Threshold of Viability
It appears generally accepted that births before 26 weeks, especially those weighing less than 750 g, are at the current threshold of viability and that these preterm infants pose a variety of complex medical, social, and ethical considerations (American College of Obstetricians and Gynecologists, 2002, 2008c). For example, Sidney Miller is a child who was born at 23 weeks, weighed 615 g, and survived but developed severe physical and mental impairment (Annas, 2004). At age 7 years, she was described as a child who "could not walk, talk, feed herself, or sit up on her own . . . was legally blind, suffered from severe mental retardation, cerebral palsy, seizures and spastic quadriparesis in her limbs." An important issue for her family was the need for a lifetime of medical care estimated to cost tens of millions of dollars
According to current guidelines developed by the American Academy of Pediatrics (Braner and co-workers, 2000), it is considered appropriate not to initiate resuscitation for infants younger than 23 weeks or those whose birthweight is less than 400 g. The involvement of the family is considered critical to the decision-making process with regard to resuscitation. Thus, infants now considered to be at the threshold of viability are those born at 22, 23, 24, or 25 weeks. These infants have been described as fragile and vulnerable because of their immature organ systems (Vohr and Allen, 2005). Moreover, they are at high risk for brain injury from hypoxic-ischemia injury and sepsis (Stoll and associates, 2004). In this setting, hypoxia and sepsis start a cascade of events that lead to brain hemorrhage, white-matter injury that causes.periventricular leukomalacia, and poor subsequent brain growth eventuating in neurodevelopmental impairment (see Chap. 29, Periventricular Leukomalacia). It is thought that because active brain development normally occurs throughout the second and third trimesters, those infants born at 22 to 25 weeks are especially vulnerable to brain injury because of their extreme immaturity.
Considerable outcome data for preterm live births between 22 and 25 weeks have become available since the last edition of this textbook. Shown in Table 36-3 are rates of overall survival as well as survival with selected complications in 250-g birthweight increments for very-low-birthweight infants. Of those with birthweights 500 to 750 g, only 55 percent survived, and most had severe complications. Survival, even with no complications apparent at initial hospital discharge, does not preclude serious developmental impairment at age 8 to 9 years (Fig. 36-3). Importantly, survival of very low-birthweight infants with and without complications of prematurity was not substantially improved when two epochs—1997 to 2002 and 1995 to 1996 are compared (Eichenwald and Stark, 2008). Saigal and Doyle (2008) collated 16 reports based on geographically defined cohorts from Australia and several European countries. Survival increased progressively from 1.7 percent at 22 weeks to 54 percent at 25 weeks. Overall, 25 percent of infants born at 22 to 25 weeks had severe neurological disabilities, and 72 percent of those with birthweights < 750 g experienced difficulty in school. Marlow and colleagues (2005) identified all infants born between 22 and 25 weeks in the United Kingdom and Ireland between March and December 1995 and examined the children at age 6 years. As shown in Table 36-4, survival was rare at 22 weeks—only 2 of 138 infants lived. This rate increased to 10 percent, 26 percent, and 43 percent, at 23, 24 and 25 weeks, respectively. Moderate to severe disability at age 6 years was identified in more than 90 percent of infants surviving following birth at 22 to 24 weeks. As shown in Figure 36-4, cognitive scores were much lower for infants born at 22 to 25 weeks compared with those of normal term births.
Importantly, female gender, singleton pregnancy, corticosteroids given for lung maturation, and higher gestational age improved the prognosis for these infants born at the threshold of viability.
Cesarean delivery at the threshold of viability is controversial. For example, if the fetus-infant is perceived to be too immature for aggressive support, then cesarean delivery for common indications such as breech presentation or nonreassuring fetal heart rate patterns might be preempted. This aside, national data clearly show a high frequency of cesarean delivery for the smallest infants (Fig. 36-5). Moreover, neonatal mortality rates in the very smallest infants—500 to 700 g—are approximately half if cesarean delivery is used compared with vaginal birth
Parkland guidelines: From an obstetrical standpoint, all fetal indications for cesarean delivery in more advanced pregnancies are practiced in women at 25 weeks. Cesarean delivery is not offered for fetal indications at 23 weeks. At 24 weeks, cesarean delivery is not offered unless fetal weight is estimated at 750 g or greater. Aggressive obstetrical management is practiced in cases of growth restriction.
Late Preterm Birth
Infants between 34 and 36 weeks account for approximately 75 percent of all preterm births, as shown in Figure 36-7, and are the fastest increasing and largest proportion of singleton preterm births in the United States
Thus, late preterm births accounted for three fourths of all preterm births. Approximately 80 percent of late preterm births were due to idiopathic spontaneous preterm labor or prematurely ruptured membranes (Fig. 36-8). Complications such as hypertension or placental accidents were implicated in approximately 20 percent of cases. Neonatal mortality rates were significantly increased in each late preterm week compared with those at 39 weeks as the referent and as shown in Figure 36-9.
Reasons for Preterm Delivery
There are four main direct reasons for preterm births in the United States:
1. Delivery for maternal or fetal indications in which labor is induced or the infant is delivered by prelabor cesarean delivery
2. Spontaneous unexplained preterm labor with intact membranes
3. Idiopathic preterm premature rupture of membranes (PPROM)
4. Twins and higher-order multifetal births.
Of preterm births, 30 to 35 percent are indicated, 40 to 45 percent are due to spontaneous preterm labor, and 30 to 35 percent follow preterm rupture of membranes. Reasons for preterm birth have multiple, often interacting, antecedents and contributing factors. This complexity has greatly confounded efforts to prevent and manage this complication. This is particularly true for preterm ruptured membranes and spontaneous preterm labor, which together lead to 70 to 80 percent of preterm births.
Medical and Obstetrical Indications
Ananth and Vintzileos (2006) used Missouri birth data from 1989 to 1997 to analyze factors leading to indicated birth before 35 weeks. Preeclampsia, fetal distress, small for gestational age, and placental abruption were the most common indications for medical intervention resulting in preterm birth. Other less common causes were chronic hypertension, placenta previa, unexplained bleeding, diabetes, renal disease, Rh isoimmunization, and congenital malformations.
Preterm Prematurely Ruptured Membranes
Defined as rupture of the membranes before labor and prior to 37 weeks, preterm premature rupture of membranes can result from a wide array of pathological mechanisms, including intra-amnionic infection. Other factors implicated include low socioeconomic status, low body mass index—less than 19.8, nutritional deficiencies, and cigarette smoking. Women with prior preterm ruptured membranes are at increased risk for recurrence during a subsequent pregnancy (Bloom and associates, 2001). Most cases of preterm rupture, however, occur without risk factors
Spontaneous Preterm Labor
Most commonly, preterm birth—up to 45 percent of cases—follows spontaneous labor. Goldenberg and colleagues (2008b) reviewed the pathogenesis of preterm labor and implicated: (1) progesterone withdrawal, (2) oxytocin initiation, and (3) decidual activation. Deviations from normal fetal growth have also been noted in spontaneous preterm labor and suggest a fetal role (Morken and co-workers, 2006).
The progesterone withdrawal theory stems from studies in sheep. As parturition nears, the fetal-adrenal axis becomes more sensitive to adrenocorticotropic hormone, increasing the secretion of cortisol (see Chap. 6, Actions of Corticotropin-Releasing Hormone on the Fetal Adrenal Gland). Fetal cortisol stimulates placental 17--hydroxylase activity, which decreases progesterone secretion and increases estrogen production. The reversal in the estrogen/progesterone ratio results in increased prostaglandin formation, which initiates a cascade that culminates in labor. In human beings, serum progesterone concentrations do not fall as labor approaches. Even so, because progesterone antagonists such as RU486 initiate preterm labor and progestational agents prevent preterm labor, decreased local progesterone concentrations may play a role.
Because intravenous oxytocin increases the frequency and intensity of uterine contractions, oxytocin is assumed to play a part in labor initiation. But serum concentrations of oxytocin do not rise before labor, and the clearance of oxytocin remains constant. Accordingly, oxytocin is an unlikely initiator.
An important pathway leading to labor initiation implicates inflammatory decidual activation. At term, such activation seems to be mediated at least in part by the fetal-decidual paracrine system and perhaps through localized decreases in progesterone concentration. In many cases of early preterm labor, however, decidual activation seems to arise in the context of intrauterine bleeding or occult intrauterine infection.
Threatened Abortion
Vaginal bleeding in early pregnancy is associated with increased adverse outcomes later
Cigarette smoking, inadequate maternal weight gain, and illicit drug use have important roles in both the incidence and outcome of low-birthweight neonates (see Chap. 14, Tobacco). In addition, Ehrenberg and colleagues (2009) found that overweight women at risk for preterm birth had lower rates of preterm delivery before 35 weeks than at-risk women with normal weight. Some of these effects are undoubtedly due to restricted fetal growth, but Hickey and colleagues (1995) linked prenatal weight gain specifically with preterm birth. Other maternal factors implicated include young or advanced maternal age, poverty, short stature, vitamin C deficiency, and occupational factors such as prolonged walking or standing, strenuous working conditions, and long weekly work hours
In the United States and in the United Kingdom, women classified as black, African-American, and Afro-Caribbean are consistently reported to be at higher risk of preterm birth.
Studies of work and physical activity related to preterm birth have produced conflicting results (Goldenberg and colleagues, 2008b). There is some evidence, however, that working long hours and hard physical labor are probably associated with increased risk of preterm birth.
Vergnes and Sixou (2007) performed a meta-analysis of 17 studies and concluded that periodontal disease was significantly associated with preterm birth—odds ratio 2.83 (CI 1.95–4.10). In an accompanying editorial, Stamilio and colleagues (2007) concluded that the data used were not robust enough to recommend screening and treatment of pregnant women.
Birth Defects
Short intervals between pregnancies have been known for some time to be associated with adverse perinatal outcomes. In a recent meta-analysis, Conde-Agudelo and co-workers (2006) reported that intervals shorter than 18 months and longer than 59 months were associated with increased risks for both preterm birth and small-for-gestational age infants. Prior Preterm Birth
A major risk factor for preterm labor is prior preterm delivery (Spong, 2007). Shown in Table 36-6 is the incidence of recurrent preterm birth in nearly 16,000 women delivered at Parkland Hospital (Bloom and associates, 2001). The risk of recurrent preterm delivery for women whose first delivery was preterm was increased threefold compared with that of women whose first neonate was born at term Self-reported coitus during early pregnancy was not associated with an increased risk of recurrent preterm birth (Yost and co-workers, 2006).
Infection: It is hypothesized that intrauterine infections trigger preterm labor by activation of the innate immune system. In this hypothesis, microorganisms elicit release of inflammatory cytokines such as interleukins and tumor necrosis factor (TNF), which in turn stimulate the production of prostaglandin and/or matrix-degrading enzymes. Prostaglandins stimulate uterine contractions, whereas degradation of extracellular matrix in the fetal membranes leads to preterm rupture of membranes. It is estimated that 25 to 40 percent of preterm births result from intrauterine infection. Potential routes of intrauterine infection are shown in Figure 36-10.
Two microorganisms, Ureaplasma urealyticum and Mycoplasma hominis, have emerged as important perinatal pathogens. Goldenberg and colleagues (2008a) reported that 23 percent of neonates born between 23 and 32 weeks have positive umbilical blood cultures for these genital mycoplasmas.
Morency and Bujold (2007) performed a meta-analysis of 61 articles and suggested that antimicrobials given in the second trimester may prevent subsequent preterm birth. Andrews and colleagues (2006) reported results of a double-blind interconceptional trial from the University of Alabama in Birmingham. A course of azithromycin plus metronidazole was given every 4 months to 241 nonpregnant women whose last pregnancy resulted in spontaneous delivery before 34 weeks. Approximately 80 percent of the women with subsequent pregnancies had received study drug within 6 months of their subsequent conception. Such interconceptional antimicrobial treatment did not reduce the rate of recurrent preterm birth. Tita and co-workers (2007) performed a subgroup analysis of these same data and concluded that such use of antimicrobials may be harmful. In another study, Goldenberg and colleagues (2006) randomized 2661 women at four African sites to placebo or metronidazole plus erythromycin between 20 and 24 weeks followed by ampicillin plus metronidazole during labor. This antimicrobial regimen did not reduce the rate of preterm birth nor that of histological chorioamnionitis.
Bacterial vaginosis has been associated with spontaneous abortion, preterm labor, preterm rupture of membranes, chorioamnionitis, and amnionic fluid infection
Diagnosis
Early differentiation between true and false labor is difficult before there is demonstrable cervical effacement and dilatation. Uterine activity alone can be misleading because of Braxton Hicks contractions, which are discussed in detail in Chapter 18, Patterns of Uterine Activity. These contractions, described as irregular, nonrhythmical, and either painful or painless, can cause considerable confusion in the diagnosis of true preterm labor. Not infrequently, women who deliver before term have uterine activity that is attributed to Braxton Hicks contractions, prompting an incorrect diagnosis of false labor. Because uterine contractions alone may be misleading, the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists (1997) had earlier proposed the following criteria to document preterm labor:
1. Contractions of four in 20 minutes or eight in 60 minutes plus progressive change in the cervix
2. Cervical dilatation greater than 1 cm
3. Cervical effacement of 80 percent or greater
In addition to painful or painless uterine contractions, symptoms such as pelvic pressure, menstrual-like cramps, watery vaginal discharge, and lower back pain have been empirically associated with impending preterm birth
Cervical Dilatation
Asymptomatic cervical dilatation after midpregnancy is suspected as a risk factor for preterm delivery, although some clinicians consider it to be a normal anatomical variant, particularly in parous women. Studies, however, have suggested that parity alone is not sufficient to explain cervical dilatation discovered early in the third trimester. Cook and Ellwood (1996) longitudinally evaluated cervical status with transvaginal sonography between 18 and 30 weeks in both nulliparous and parous women who all subsequently gave birth at term. Cervical length and diameter were identical in both groups throughout these critical weeks.
Iams and co-workers (1996) measured cervical length at approximately 24 weeks and again at 28 weeks in 2915 women not at risk for preterm birth. The mean cervical length at 24 weeks was approximately 35 mm, and those women with progressively shorter cervices experienced increased rates of preterm birth.
These investigators correlated sonographic cervical length, funneling, and prior history of preterm birth with delivery before 35 weeks. Funneling was defined as bulging of the membranes into the endocervical canal and protruding at least 25 percent of the entire cervical length (Fig. 36-11). As shown in Figure 36-12, a short cervix by itself was the poorest predictor of preterm birth, whereas funneling plus a history of prior preterm birth was highly predictive.
Incompetent Cervix
Cervical incompetence is a clinical diagnosis characterized by recurrent, painless cervical dilatation and spontaneous midtrimester birth in the absence of spontaneous membrane rupture, bleeding, or infection
Fibronectin: Lockwood and co-workers (1991) reported that fibronectin detection in
Prevention of Preterm Birth
Weekly intramuscular injections of either inert oil or 17-hydroxyprogesterone caproate were given from 16 through 36 weeks. Rates of delivery before 37, 35, and 32 weeks were all significantly reduced by progestin therapy. But similar studies of 17-hydroxyprogesterone caproate in both twins and triplets done by the Network showed no improvement in preterm birth rates. In the first study, 142 women with a prior preterm birth, prophylactic cervical cerclage, or uterine malformation were randomly assigned to daily 100-mg progesterone or placebo suppositories. Progesterone suppositories were associated with a significant reduction in births before 34 weeks.
At this time, the American College of Obstetricians and Gynecologists (2008c) has concluded that progesterone therapy should be limited to women with a documented history of a previous spontaneous birth at less than 37 weeks. Further studies are needed as to optimal preparation, dosage, and route of administration.
Cervical Cerclage
There are at least three circumstances when cerclage placement may be used to prevent preterm birth. First, cerclage may be used in women who have a history of recurrent midtrimester losses and who are diagnosed with an incompetent cervix (see Chap. 9, Incompetent Cervix). A second circumstance is in women identified during sonographic examination to have a short cervix. The third indication is "rescue" cerclage, done emergently when cervical incompetence is recognized in the women with threatened preterm labor
Management of Preterm Rupture of Membranes and Preterm Labor
Diagnosis of Preterm Prematurely Ruptured Membranes – check
Although this complication was identified in only 1.7 percent of pregnancies, it contributed to 20 percent of all perinatal deaths. By the time they presented, 75 percent of the women were already in labor, 5 percent were delivered for other complications, and another 10 percent were delivered within 48 hours. The time from preterm ruptured membranes to delivery is inversely proportional to the gestational age when rupture occurs (Carroll and associates, 1995). As shown in Figure 36-15, very few days were gained when membranes ruptured during the third trimester compared with midpregnancy
Most clinicians hospitalize women with preterm ruptured membranes. Concerns about the costs of lengthy hospitalizations are usually moot, because most women enter labor within a week or less after membrane rupture. Carlan and co-workers (1993) randomly assigned 67 women with ruptured membranes to home or hospital management. No benefits were found for hospitalization, and maternal hospital stays were reduced by 50 percent in those sent home.
Fetal lung maturity, as evidenced by mature surfactant profiles, was present in all cases. Intentional delivery reduced the length of maternal hospitalization and also reduced infection rates in both mothers and neonates. Cox and Leveno (1995) similarly apportioned 129 women between 30 and 34 weeks. Fetal lung maturity was not assessed. One fetal death resulted from sepsis in the pregnancies managed expectantly. Among those intentionally delivered, there were three neonatal deaths—two from sepsis and one from pulmonary hypoplasia. Thus, neither management approach proved to be superior.
Despite extensive literature concerning expectant management of preterm ruptured membranes, tocolysis has been used in few studies. In randomized studies, women were assigned to receive either tocolysis or expectant management. The investigators concluded that active interventions did not improve perinatal outcomes.
McElrath and associates (2002) studied 114 women with cerclage in place who later had ruptured membranes prior to 34 weeks. They were compared with 288 controls who had not received a cerclage. Pregnancy outcomes were equivalent in both groups.
Risks of Expectant Management
Maternal and fetal risks vary with the gestational age at membrane rupture. Morales and Talley (1993b) expectantly managed 94 singleton pregnancies with ruptured membranes prior to 25 weeks. The average time gained was 11 days. Although 41 percent of infants survived to age 1 year, only 27 percent were neurologically normal. Similar results were reported by Farooqi (1998) and Winn (2000) and their associates. Lieman and colleagues (2005) found no improved neonatal outcomes with expectant management beyond 33 weeks. In contrast, McElrath and co-workers (2003) found that prolonged latency after membrane rupture was not associated with an increased incidence of fetal neurological damage
Virtually all women with oligohydramnios delivered before 25 weeks, whereas 85 percent with adequate amnionic fluid volume were delivered in the third trimester. Carroll and colleagues (1995) observed no cases of pulmonary hypoplasia in fetuses born after membrane rupture at 24 weeks or beyond. This suggests that 23 weeks or less is the threshold for development of lung hipoplasia.
Corioamnionitis: Clinical Chorioamnionitis
Most authors report that prolonged membrane rupture is associated with increased fetal and maternal sepsis (Ho and colleagues, 2003). If chorioamnionitis is diagnosed, prompt efforts to effect delivery, preferably vaginally, are initiated. Fever is the only reliable indicator for this diagnosis, and temperature of 38°C (100.4°F) or higher accompanying ruptured membranes implies infection. Maternal leukocytosis alone has not been found to be reliable. During expectant management, monitoring for sustained maternal or fetal tachycardia, for uterine tenderness, and for a malodorous vaginal discharge is warranted
Accelerated Pulmonary Maturation
A variety of clinical events—some well defined—were once proposed to accelerate fetal surfactant production (Gluck, 1979). These included chronic renal or cardiovascular disease, hypertensive disorders, heroin addiction, fetal-growth restriction, placental infarction, chorioamnionitis, and preterm ruptured membranes. Although this view was widely held for many years, subsequent observations do not support this association
Antimicrobial therapy: To further address this issue, the Maternal-Fetal Medicine Units Network designed a trial to study expectant management combined with a 7-day treatment of ampicillin, amoxicillin plus erythromycin, or placebo. The women had membrane rupture between 24 and 32 weeks. Neither tocolytics nor corticosteroids were given. Antimicrobial-treated women had significantly fewer newborns with respiratory distress syndrome, necrotizing enterocolitis, and composite adverse outcomes (Mercer and colleagues, 1997). The latency period was significantly longer. Specifically, 50 percent of women given an antimicrobial regimen remained undelivered after 7 days of treatment compared with only 25 percent of those given placebo. There was also significant prolongation of pregnancy at 14 and 21 days. Cervicovaginal group B streptococcal colonization did not alter these results
Corticosteroids: Since then, a number of meta-analyses have addressed this issue, and according to the American College of Obstetricians and Gynecologists (2007), single-dose therapy is recommended from 24 to 32 weeks. There is no consensus regarding treatment between 32 and 34 weeks. They are not recommended prior to 24 weeks.
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Preterm Labor with Intact Membranes
Women with signs and symptoms of preterm labor with intact membranes are managed much the same as described above for those with preterm ruptured membranes. The cornerstone of treatment is to avoid delivery prior to 34 weeks, if possible. Drugs used to abate or suppress preterm uterine contractions are subsequently discussed
The American College of Obstetricians and Gynecologists (2003) has concluded that there is no evidence to support routine amniocentesis to identify infection.
The issue of the fetal and infant safety with single versus repeat courses of corticosteroids for lung maturation has been the topic of two major trials. Although both found repeated courses to be beneficial in reducing neonatal respiratory morbidity rates, the long-term consequences were much different. Specifically, Crowther and colleagues (2007) studied outcomes in 982 women from the Australian Collaborative Study. These women were given a single weekly dose of 11.4 mg of betamethasone. These investigators found no adverse effects in the infants followed to age 2 years. Wapner and colleagues (2007) studied infants born to 495 women in a Network study who were randomized to receive two weekly 12-mg betamethasone doses given 24 hours apart. They were concerned by their finding of a nonsignificant increase in cerebral palsy rates in infants exposed to repeated courses. The twice-as-large betamethasone dose in the Network study was worrisome because there is some experimental evidence to support the view that adverse corticosteroid effects are dose dependent. Bruschettini and colleagues (2006) studied the equivalent of 12-mg versus 6-mg betamethasone given to pregnant cats. They reported that the lower dose had less severe effects on somatic growth without affecting cell proliferation in the fetal brain.
"Rescue" Therapy
This refers to administration of a repeated corticosteroid dose when delivery becomes imminent and more than 7 days have elapsed since the initial dose. The 2000 Consensus Development Conference recommended that rescue therapy should not be routinely used and that it should be reserved for clinical trials. The first randomized trial reported by Peltoniemi and colleagues (2007) allocated 326 women to placebo or 12-mg betamethasone single-dose rescue regimens. Paradoxically, they found that the rescue dose of betamethasone increased the risk of respiratory distress syndrome! Subsequently, the American College of Obstetricians and Gynecologists (2008a) also recommended such therapy for trials
Elimian and co-workers (2007) randomized 299 women between 24 and 33 weeks in a double-blinded trial of betamethasone versus dexamethasone. These two drugs were comparable in reducing the rates of major neonatal morbidities in preterm infants.
Antimicrobials: As with preterm ruptured membranes, antimicrobials have been given to arrest preterm labor. Here too, results have been disappointing. A Cochrane meta-analysis by King and Flenady (2000) of 10 randomized trials found no difference in the rates of newborn respiratory distress syndrome or of sepsis between placebo- and antimicrobial-treated groups.
Emergency or Rescue Cerclage There is support for the concept that cervical incompetence and preterm labor are part of a spectrum leading to preterm delivery. Consequently, investigators have evaluated the role of cerclage done after preterm labor begins to manifest clinically. Harger (1983) concluded that if cervical incompetence is recognized with threatened preterm labor, emergency cerclage can be attempted, albeit with an appreciable risk of infection and pregnancy loss. Althuisius and associates (2003) randomly assigned 23 women with cervical incompetence prior to 27 weeks to bed rest, with or without emergency McDonald cerclage. Delivery delay was significantly greater in the cerclage group compared with that of bed rest alone—54 versus 24 days.
Inhibition of Preterm Labor
Although a number of drugs and other interventions have been used to prevent or inhibit preterm labor, none has been shown to be completely effective. The American College of Obstetricians and Gynecologists (2007) has concluded that tocolytic agents do not markedly prolong gestation, but may delay delivery in some women for at least 48 hours. This may facilitate transport to a regional obstetrical center and allow time for administration of corticosteroid therapy. It is estimated that 72,733 women giving birth in the United States in 2006 received tocolytic drugs (Martin and colleagues, 2009). The rate of tocolysis use was 1.7 percent in 2006, a rate that has fluctuated only slightly since 1996. Norwitz and co-workers (2004) performed an interactive survey of tocolytic treatment at the 2004 Annual Meeting of the Society of Maternal-Fetal Medicine. As shown in Figure 36-16, intravenous magnesium sulfate was the most commonly used tocolytic—about 70 percent of respondents. Approximately the same proportion of respondents would not prescribe maintenance therapy
Bed Rest
Hydration and Sedation
Helfgott and associates (1994) compared hydration and sedation with bed rest in a randomized trial of 119 women with threatened preterm labor. Women who were randomly selected to receive 500 mL of crystalloid over 30 minutes and 8 to 12 mg of intramuscular morphine sulfate had outcomes similar to those prescribed bed rest. Although women with preterm contractions treated with 0.25-mg subcutaneous terbutaline may have contractions that cease more quickly and may be discharged significantly earlier compared with women not treated, pregnancy outcomes are similar
-Adrenergic Receptor Agonists: A number of compounds react with -adrenergic receptors to reduce intracellular ionized calcium levels and prevent activation of myometrial contractile proteins (see Chap. 6, Regulation of Myometrial Contraction and Relaxation). In the United States, ritodrine and terbutaline have been used in obstetrics, but only ritodrine had been approved for preterm labor by the Food and Drug Administration.
Ritodrine: In a multicenter trial, neonates whose mothers were treated with ritodrine for threatened preterm labor had lower rates of death and respiratory distress. They also achieved a gestational age of 36 weeks or a birthweight of 2500 g more often than did those of untreated mothers (
The infusion of -agonists has resulted in frequent and at times, serious and fatal side effects. Pulmonary edema is a special concern, and its contribution to morbidity is discussed in Chapter 42. Tocolysis was the third most common cause of acute respiratory distress and death in pregnant women during a 14-year period in Mississippi (Perry and associates, 1996). The cause of pulmonary edema is multifactorial, and risk factors include tocolytic therapy with -agonists, multifetal gestation, concurrent corticosteroid therapy, tocolysis for more than 24 hours, and large intravenous crystalloid volume infusion. Because -agonists cause retention of sodium and water, with time—usually 24 to 48 hours, these can cause volume overload (Hankins and colleagues, 1988). The drugs have been implicated as a cause of increased capillary permeability, disturbance of cardiac rhythm, and myocardial ischemia. Finally, maternal sepsis—a relatively common occurrence with preterm labor—appreciably increases this risk
Terbutaline: Elliott and co-workers (2004) have used continuous terbutaline subcutaneous infusion in 9359 patients and reported that only 12 women experienced severe adverse events—primarily pulmonary edema. Two randomized trials found no benefit for terbutaline pump therapy. Oral terbutaline therapy to prevent preterm delivery has also not been effective.
Magnesium Sulfate
Ionic magnesium in a sufficiently high concentration can alter myometrial contractility. Its role is presumably that of a calcium antagonist. Clinically, magnesium in pharmacological doses may inhibit labor. Steer and Petrie (1977) concluded that intravenously administered magnesium sulfate—a 4-g loading dose followed by a continuous infusion of 2 g/hr—usually arrests labor. Women given magnesium sulfate must be monitored closely for evidence of hypermagnesemia. PE in 8.5%. These women were at appreciable risk, and few reached 33 weeks. Magnesium-treated women and their fetuses had identical outcomes compared with those given placebo. Because of these findings, this method of tocolysis was abandoned at Parkland Hospital.
In intriguing reports, very-low-birthweight neonates whose mothers were treated with magnesium sulfate for preterm labor or preeclampsia were found to have a reduced incidence of cerebral palsy at 3 years (Grether and associates, 2000; Nelson and Grether, 1995). This was logical, as magnesium has been shown in adults to stabilize intracranial tone, minimize fluctuations in cerebral blood flow, reduce reperfusion injury, and block calcium-mediated intracellular damage (Aslanyan and colleagues, 2007; Marret and associates, 1995). Magnesium reduces synthesis of cytokines and bacterial endotoxins and thus, also may minimize the inflammatory effects of infection (Nelson and Grether, 1997.
This trial was interpreted differently depending upon statistical philosophy. Those who disdain subgroup analysis opted to interpret these findings to mean that magnesium sulfate infusion prevents cerebral palsy regardless of the gestational age at which the therapy is given. Those with a differing view concluded that this trial only supports use of magnesium sulfate for prevention of cerebral palsy before 28 weeks. What is certain, however, is that maternally administered magnesium sulfate infusions cannot be implicated in increased perinatal deaths as reported by Mittendorf and colleagues (1997.
Because of these findings, at Parkland Hospital, our policy is to give magnesium sulfate for threatened preterm delivery from 24 to 28 completed weeks. At the University of Alabama at Birmingham Hospital, we administer neuroprotection magnesium from 23 to 32 completed weeks. In both, a 6-g loading dose is followed by an infusion of 2 g per hour for at least 12 hours.
Morales and co-workers (1989, 1993a), however, compared indomethacin with either ritodrine or magnesium sulfate and found no difference in their efficacy to forestall preterm delivery. Berghella and colleagues (2006) reviewed four trials of indomethacin given to women with sonographically short cervices and found such therapy to be ineffective.
Indomethacin is administered orally or rectally. A dose of 50 to 100 mg is followed at 8-hour intervals not to exceed a total 24-hour dose of 200 mg. Serum concentrations usually peak 1 to 2 hours after oral administration, whereas levels after rectal administration peak slightly sooner. Most studies have limited indomethacin use to 24 to 48 hours because of concerns of oligohydramnios, which can develop with these doses. If amnionic fluid is monitored, oligohydramnios can be detected early, and it is reversible with discontinuation of indomethacin
Case-control studies have been performed to assess neonatal effects of indomethacin exposure given for preterm labor. In a study of neonates born before 30 weeks, Norton and associates (1993) identified necrotizing enterocolitis in 30 percent of 37 indomethacin-exposed newborns compared with 8 percent of 37 control newborns. Higher incidences of intraventricular hemorrhage and patent ductus arteriosus were also documented in the indomethacin group. The impact of treatment duration and its timing in relation to delivery were not reported. In contrast, several investigators have challenged the association between indomethacin exposure and necrotizing enterocolitis (Muench and co-workers, 2001; Parilla and colleagues, 2000.
Using the Cochrane Database, Keirse (1995a) compared nifedipine and -agonists and concluded that although nifedipine treatment reduced births of neonates weighing less than 2500 g, significantly more of these were admitted for intensive care. Other investigators have also concluded that calcium-channel blockers, especially nifedipine, are safer and more effective tocolytic agents than are -agonists (King and colleagues, 2003; Papatsonis and co-workers, 1997). Lyell and colleagues (2007) randomized 192 women at 24 to 33 weeks to either magnesium sulfate or nifedipine and found no substantial differences in efficacy or adverse effects. Finally, oral nifedipine does not significantly prolong pregnancy in women initially treated with intravenous magnesium sulfate for preterm labor (Carr and colleagues, 1999).
The combination of nifedipine with magnesium for tocolysis is potentially dangerous. Ben-Ami and co-workers (1994) and Kurtzman and associates (1993) reported that nifedipine enhances neuromuscular blocking effects of magnesium that can interfere with pulmonary and cardiac function
In randomized clinical trials, however, atosiban failed to improve relevant neonatal outcomes and was linked with significant neonatal morbidity.
In many women, tocolytics stop contractions temporarily but rarely prevent preterm birth. In a meta-analysis of tocolytic therapy, Gyetvai and colleagues (1999) concluded that although delivery may be delayed long enough for administration of corticosteroids, treatment does not result in improved perinatal outcome. Berkman and associates (2003) reviewed 60 reports and concluded that tocolytic therapy can prolong gestation, but that 
-agonists are not better than other drugs and pose potential maternal danger. They also concluded that there are no benefits of maintenance tocolytic therapy.
As a general rule, if tocolytics are given, they should be given concomitantly with corticosteroids. The gestational age range for their use is debatable, but because corticosteroids are not generally used after 33 weeks and because the perinatal outcomes in preterm neonates are generally good after this time, most practitioners do not recommend use of tocolytics at or after 33 weeks (
Recommended Management of Preterm Labor
The following considerations should be given to women in preterm labor:
1. Confirmation of preterm labor as detailed in Diagnosis
2. For pregnancies less than 34 weeks in women with no maternal or fetal indications for delivery, close observation with monitoring of uterine contractions and fetal heart rate is appropriate. Serial examinations are done to assess cervical changes
3. For pregnancies less than 34 weeks, corticosteroids are given for enhancement of fetal lung maturation
4. Consideration is given for maternal magnesium sulfate infusion for 12 to 24 hours to afford fetal neuroprotection
5. For pregnancies less than 34 weeks in women who are not in advanced labor, some practitioners believe it is reasonable to attempt inhibition of contractions to delay delivery while the women are given corticosteroid therapy and group B streptococcal prophylaxis. Although tocolytic drugs are not used at Parkland Hospital, they are given at University of Alabama at Birmingham Hospital
6. For pregnancies at 34 weeks or beyond, women with preterm labor are monitored for labor progression and fetal well-being
7. For active labor, an antimicrobial is given for prevention of neonatal group B streptococcal infection
Group B streptococcal infections are common and dangerous in the preterm neonate. Tachycardia in preterm fetus-->> sepsis.
Malloy and colleagues (1991) analyzed 1765 newborns with birthweights less than 1500 g and found that cesarean delivery did not lower the risk of mortality or intracranial hemorrhage. Anderson and co-workers (1988), however, made an interesting observation regarding the role of cesarean delivery in the prevention of neonatal intracranial hemorrhages. These hemorrhages related to whether or not the fetus had been subjected to the active phase of labor. They emphasized that avoidance of active-phase labor is impossible in most preterm births because the route of delivery cannot be decided until the active phase of labor is firmly established
Mechanisms of Labor
At the onset of labor, the position of the fetus with respect to the birth canal is critical to the route of delivery. Thus, fetal position within the uterine cavity should be determined at the onset of labor
Fetal Lie
The relation of the fetal long axis to that of the mother is termed fetal lie and is either longitudinal or transverse. Occasionally, the fetal and the maternal axes may cross at a 45-degree angle, forming an oblique lie, which is unstable and always becomes longitudinal or transverse during labor. A longitudinal lie is present in greater than 99 percent of labors at term. Predisposing factors for transverse lies include multiparity, placenta previa, hydramnios, and uterine anomalies
Fetal Presentation
Cephalic Presentation
Such presentations are classified according to the relationship between the head and body of the fetus (Fig. 17-1). Ordinarily, the head is flexed sharply so that the chin is in contact with the thorax. The occipital fontanel is the presenting part, and this presentation is referred to as a vertex or occiput presentation. Much less commonly, the fetal neck may be sharply extended so that the occiput and back come in contact, and the face is foremost in the birth canal—face presentation (see Fig. 20-6). The fetal head may assume a position between these extremes, partially flexed in some cases, with the anterior (large) fontanel, or bregma, presenting—sinciput presentation—or partially extended in other cases, to have a brow presentation (see Fig. 20-8). These latter two presentations are usually transient. As labor progresses, sinciput and brow presentations almost always convert into vertex or face presentations by neck flexion or extension, respectively. Failure to do so can lead to dystocia
The term fetus usually presents with the vertex, most logically because the uterus is piriform or pear shaped. Although the fetal head at term is slightly larger than the breech, the entire podalic pole of the fetus—that is, the breech and its flexed extremities—is bulkier and more mobile than the cephalic pole. The cephalic pole is composed of the fetal head only. Until approximately 32 weeks, the amnionic cavity is large compared with the fetal mass, and there is no crowding of the fetus by the uterine walls. Subsequently, however, the ratio of amnionic fluid volume decreases relative to the increasing fetal mass. As a result, the uterine walls are apposed more closely to the fetal parts.
If presenting by the breech, the fetus often changes polarity to make use of the roomier fundus for its bulkier and more mobile podalic pole. As discussed in Chapter 24 (see Fig. 24-1), the incidence of breech presentation decreases with gestational age. It is approximately 25 percent at 28 weeks, 17 percent at 30 weeks, 11 percent at 32 weeks, and then decreases to approximately 3 percent at term. The high incidence of breech presentation in hydrocephalic fetuses is in accord with this theory, because in this circumstance, the fetal cephalic pole is larger than its podalic pole.
In the later months of pregnancy the fetus assumes a characteristic posture described as attitude or habitus (see Fig. 17-1). As a rule, the fetus forms an ovoid mass that corresponds roughly to the shape of the uterine cavity. The fetus becomes folded or bent upon itself in such a manner that the back becomes markedly convex; the head is sharply flexed so that the chin is almost in contact with the chest; the thighs are flexed over the abdomen; and the legs are bent at the knees. In all cephalic presentations, the arms are usually crossed over the thorax or become parallel to the sides. The umbilical cord lies in the space between them and the lower extremities. This characteristic posture results from the mode of fetal growth and its accommodation to the uterine cavity.
Abnormal exceptions to this attitude occur as the fetal head becomes progressively more
Fetal Position
Approximately two thirds of all vertex presentations are in the left occiput position, and one
Diagnosis of Fetal Presentation and Position
Several methods can be used to diagnose fetal presentation and position. These include abdominal palpation, vaginal examination, auscultation, and, in certain doubtful cases, sonography. Occasionally plain radiographs, computed tomography, or magnetic resonance imaging may be used.
Abdominal Palpation—Leopold Maneuvers
Abdominal examination can be conducted systematically employing the four maneuvers described by Leopold in 1894 and shown in Figure 17-8. The mother lies supine and comfortably positioned with her abdomen bared. These maneuvers may be difficult if not impossible to perform and interpret if the patient is obese, if there is excessive amnionic fluid, or if the placenta is anteriorly implanted.
1. The first maneuver permits identification of which fetal pole—that is, cephalic or podalic—occupies the uterine fundus. The breech gives the sensation of a large, nodular mass, whereas the head feels hard and round and is more mobile and ballottable
2. Performed after determination of fetal lie, the second maneuver is accomplishe
d as the palms are placed on either side of the maternal abdomen, and gentle but deep pressure is exerted. On one side, a hard, resistant structure is felt—the back. On the other, numerous small, irregular, mobile parts are felt—the fetal extremities. By noting whether the back is directed anteriorly, transversely, or posteriorly, the orientation of the fetus can be determined
3. The third maneuver is performed by grasping with the thumb and fingers of one hand the lower portion of the maternal abdomen just above the symphysis pubis. If the presenting part is not engaged, a movable mass will be felt, usually the head. The differentiation between head and breech is made as in the first maneuver. If the presenting part is deeply engaged, however, the findings from this maneuver are simply indicative that the lower fetal pole is in the pelvis, and details are then defined by the fourth maneuver
4. To perform the fourth maneuver, the examiner faces the mother's feet and, with the tips of the first three fingers of each hand, exerts deep pressure in the direction of the axis of the pelvic inlet. In many instances, when the head has descended into the pelvis, the anterior shoulder may be differentiated readily by the third maneuver.
Vaginal Examination
Before labor, the diagnosis of fetal presentation and position by vaginal examination is often inconclusive because the presenting part must be palpated through a closed cervix and lower uterine segment. With the onset of labor and after cervical dilatation, vertex presentations and their positions are recognized by palpation of the various fetal sutures and fontanels. Face and breech presentations are identified by palpation of the facial features and the fetal sacrum, respectively.
In attempting to determine presentation and position by vaginal examination, it is advisable to pursue a definite routine, comprising four movements:
1. The examiner inserts two fingers into the vagina and the presenting part is found. Differentiation of vertex, face, and breech is then accomplished readily
2. If the vertex is presenting, the fingers are directed posteriorly and then swept forward over the fetal head toward the maternal symphysis (F
ig. 17-9). During this movement, the fingers necessarily cross the sagittal suture and its course is delineated
3. The positions of the two fontanels then are ascertained. The fingers are passed to the most anterior extension of the sagittal suture, and the fontanel encountered there is examined and identified. Then, with a sweeping motion, the fingers pass along the suture to the other end of the head until the other fontanel is felt and differentiated (Fig. 17-10)
4. The station, or extent to which the presenting part has descended into the pelvis, can also be established at this time (see Cervical Examination). Using these maneuvers, the various sutures and fontanels are located readily
Mechanisms of Labor with Occiput Anterior Presentation
In most cases, the vertex enters the pelvis with the sagittal suture lying in the transverse pelvic diameter. The fetus enters the pelvis in the left occiput transverse (LOT) position in 40 percent of labors and in the right occiput transverse (ROT) position in 20 percent (Caldwell and associates, 1934). In occiput anterior positions—LOA or ROA—the head either enters the pelvis with the occiput rotated 45 degrees anteriorly from the transverse position, or subsequently does so. The mechanism of labor in all these presentations is usually similar.
The positional changes in the presenting part required to navigate the pelvic canal constitute the mechanisms of labor. The cardinal movements of labor are engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion (Fig. 17-11). During labor, these movements not only are sequential but also show great temporal overlap. For example, as part of engagement, there is both flexion and descent of the head. It is impossible for the movements to be completed unless the presenting part descends simultaneously. Concomitantly, uterine contractions effect important modifications in fetal attitude, or habitus, especially after the head has descended into the pelvis. These changes consist principally of fetal straightening, with loss of dorsal convexity and closer application of the extremities to the body. As a result, the fetal ovoid is transformed into a cylinder, with the smallest possible cross section typically passing through the birth canal
Engagement
Although the fetal head tends to accommodate to the transverse axis of the pelvic inlet, the sagittal suture, while remaining parallel to that axis, may not lie exactly midway between the symphysis and the sacral promontory. The sagittal suture frequently is deflected either posteriorly toward the promontory or anteriorly toward the symphysis (Fig. 17-12). Such lateral deflection to a more anterior or posterior position in the pelvis is called asynclitism. If the sagittal suture approaches the sacral promontory, more of the anterior parietal bone presents itself to the examining fingers, and the condition is called anterior asynclitism. If, however, the sagittal suture lies close to the symphysis, more of the posterior parietal bone will present, and the condition is called posterior asynclitism. With extreme posterior asynclitism, the posterior ear may be easily palpated.
Moderate degrees of asynclitism are the rule in normal labor. However, if severe, the condition is a common reason for cephalopelvic disproportion even with an otherwise normal-sized pelvis. Successive shifting from posterior to anterior asynclitism aids descent.
This movement is the first requisite for birth of the newborn. In nulliparas, engagement may take place before the onset of labor, and further descent may not follow until the onset of the second stage. In multiparous women, descent usually begins with engagement. Descent is brought about by one or more of four forces: (1) pressure of the amnionic fluid, (2) direct pressure of the fundus upon the breech with contractions, (3) bearing-down efforts of maternal abdominal muscles, and (4) extension and straightening of the fetal body.
As soon as the descending head meets resistance, whether from the cervix, walls of the pelvis, or pelvic floor, then flexion of the head normally results. In this movement, the chin is brought into more intimate contact with the fetal thorax, and the appreciably shorter suboccipitobregmatic diameter is substituted for the longer occipitofrontal diameter
Internal Rotation
This movement consists of a turning of the head in such a manner that the occiput gradually moves toward the symphysis pubis anteriorly from its original position or less commonly, posteriorly toward the hollow of the sacrum (Figs. 17-15, 17-16, and 17-17). Internal rotation is essential for the completion of labor, except when the fetus is unusually small.
Calkins (1939) studied more than 5000 women in labor to the time of internal rotation. He concluded that in approximately two thirds, internal rotation is completed by the time the head reaches the pelvic floor; in about another fourth, internal rotation is completed very shortly after the head reaches the pelvic floor; and in the remaining 5 percent, anterior rotation does not take place. When the head fails to turn until reaching the pelvic floor, it typically rotates during the next one or two contractions in multiparas. In nulliparas, rotation usually occurs during the next three to five contractions.
Extension
After internal rotation, the sharply flexed head reaches the vulva and undergoes extension. If the sharply flexed head, on reaching the pelvic floor, did not extend but was driven farther downward, it would impinge on the posterior portion of the perineum and would eventually be forced through the tissues of the perineum. When the head presses upon the pelvic floor, however, two forces come into play. The first force, exerted by the uterus, acts more posteriorly, and the second, supplied by the resistant pelvic floor and the symphysis, acts more anteriorly. The resultant vector is in the direction of the vulvar opening, thereby causing head extension. This brings the base of the occiput into direct contact with the inferior margin of the symphysis pubis
With progressive distension of the perineum and vaginal opening, an increasingly larger portion of the occiput gradually appears. The head is born as the occiput, bregma, forehead, nose, mouth, and finally the chin pass successively over the anterior margin of the perineum (see Fig. 17-17). Immediately after its delivery, the head drops downward so that the chin lies over the maternal anus.
External Rotation
Expulsion
Almost immediately after external rotation, the anterior shoulder appears under the symphysis pubis, and the perineum soon becomes distended by the posterior shoulder. After delivery of the shoulders, the rest of the body quickly passes
In approximately 20 percent of labors, the fetus enters the pelvis in an occiput posterior (OP) position. The right occiput posterior (ROP) is slightly more common than the left (LOP) (Caldwell and associates, 1934). It appears likely from radiographic evidence that posterior positions are more often associated with a narrow forepelvis. They also are more commonly seen in association with anterior placentation (Gardberg and Tuppurainen, 1994a).
In most occiput posterior presentations, the mechanism of labor is identical to that observed in the transverse and anterior varieties, except that the occiput has to internally rotate to the symphysis pubis through 135 degrees, instead of 90 and 45 degrees, respectively (see Fig. 17-17).
With effective contractions, adequate flexion of the head, and a fetus of average size, most posteriorly positioned occiputs rotate promptly as soon as they reach the pelvic floor, and labor is not lengthened appreciably. In perhaps 5 to 10 percent of cases, however, rotation may be incomplete or may not take place at all, especially if the fetus is large (Gardberg and Tuppurainen, 1994b). Poor contractions, faulty flexion of the head, or epidural analgesia, which diminishes abdominal muscular pushing and relaxes the muscles of the pelvic floor, may predispose to incomplete rotation. If rotation is incomplete, transverse arrest may result. If no rotation toward the symphysis takes place, the occiput may remain in the direct occiput posterior position, a condition known as persistent occiput posterior. Both persistent occiput posterior and transverse arrest represent deviations from the normal mechanisms of labor and are considered further in Chapter 20
Caput Succedaneum
In vertex presentations, the fetal head changes shape as the result of labor forces. In prolonged labors before complete cervical dilatation, the portion of the fetal scalp immediately over the cervical os becomes edematous (see Fig. 29-12). This swelling known as the caput succedaneum (Figs. 17-18 and 17-19). It usually attains a thickness of only a few millimeters, but in prolonged labors it may be sufficiently extensive to prevent the differentiation of the various sutures and fontanels. More commonly, the caput is formed when the head is in the lower portion of the birth canal and frequently only after the resistance of a rigid vaginal outlet is encountered. Because it develops over the most dependent area of the head, one may deduce the original fetal head position by noting the location of the caput succedaneum
Molding
The change in fetal head shape from external compressive forces is referred to as molding. Possibly related to Braxton Hicks contractions, some molding develops before labor. Most studies indicate that there is seldom overlapping of the parietal bones. A "locking" mechanism at the coronal and lambdoidal connections actually prevents such overlapping (Carlan and colleagues, 1991). Molding results in a shortened suboccipitobregmatic diameter and a lengthened mentovertical diameter. These changes are of greatest importance in women with contracted pelves or asynclitic presentations. In these circumstances, the degree to which the head is capable of molding may make the difference between spontaneous vaginal delivery and an operative delivery. Some older literature cited severe head molding as a cause for possible cerebral trauma. Because of the multitude of associated factors, for example, prolonged labor with fetal sepsis and acidosis, it is impossible to link molding to any alleged fetal or neonatal neurological sequelae. Most cases of molding resolve within the week.
Characteristics of Normal Labor
The greatest impediment to understanding normal labor is recognizing its start. The strict definition of labor—uterine contractions that bring about demonstrable effacement and dilatation of the cervix—does not easily aid the clinician in determining when labor has actually begun, because this diagnosis is confirmed only retrospectively. Several methods may be used to define its start. One defines onset as the clock time when painful contractions become regular. Unfortunately, uterine activity that causes discomfort, but that does not represent true labor, may develop at any time during pregnancy. False labor often stops spontaneously, or it may proceed rapidly into effective contractions.
In the United States, admission for labor is frequently based on the extent of dilatation accompanied by painful contractions. When a woman presents with intact membranes, a cervical dilatation of 3 to 4 cm or greater is presumed to be a reasonably reliable threshold for the diagnosis of labor. In this case, labor onset commences with the time of admission. This presumptive method obviates many of the uncertainties in diagnosing labor during earlier stages of cervical dilatation
1º stage of labor:
1. During the preparatory division, although the cervix dilates little, its connective tissue components change considerably (see Chap. 6, Phase 2 of Parturition: Preparation for Labor). Sedation and conduction analgesia are capable of arresting this division of labor.
2. The dilatational division, during which dilatation proceeds at its most rapid rate, is unaffected by sedation or conduction analgesia.
3. The pelvic division commences with the deceleration phase of cervical dilatation. The classic mechanisms of labor that involve the cardinal fetal movements of the cephalic presentation—engagement, flexion, descent, internal rotation, extension, and external rotation—take place principally during the pelvic division. In actual practice, however, the onset of the pelvic division is seldom clearly identifiable
As shown in Figure 17-20, the pattern of cervical dilatation during the preparatory and dilatational divisions of normal labor is a sigmoid curve. Two phases of cervical dilatation are defined. The latent phase corresponds to the preparatory division, and the active phase, to the dilatational division. Friedman subdivided the active phase into the acceleration phase, the phase of maximum slope, and the deceleration phase (Fig. 17-21).
Latent Phase
The onset of latent labor, as defined by Friedman (1972), is the point at which the mother perceives regular contractions. The latent phase for most women ends at between 3 and 5 cm of dilatation. This threshold may be clinically useful, for it defines cervical dilatation limits beyond which active labor can be expected
Prolonged Latent Phase
Active Labor
As shown in Figure 17-22, the progress of labor in nulliparous women has particular significance because these curves all reveal a rapid change in the slope of cervical dilatation rates between 3 and 5 cm. Thus, cervical dilatation of 3 to 5 cm or more, in the presence of uterine contractions, can be taken to reliably represent the threshold for active labor. Similarly, these curves provide useful guideposts for labor management
Turning again to Friedman (1955), the mean duration of active-phase labor in nulliparas was 4.9 hours. But the standard deviation of 3.4 hours is large; hence, the active phase was reported to have a statistical maximum of 11.7 hours. Indeed, rates of cervical dilatation ranged from a minimum of 1.2 up to 6.8 cm/hr. Friedman (1972) also found that multiparas progress somewhat faster in active-phase labor, with a minimum normal rate of 1.5 cm/hr. His analysis of active-phase labor concomitantly describes rates of fetal descent and cervical dilatation (see Fig. 17-20). Descent begins in the later stage of active dilatation, commencing at 7 to 8 cm in nulliparas and becoming most rapid after 8 cm.
Abnormalities in this labor phase are common. Sokol and co-workers (1977) reported that 25 percent of nulliparous and 15 percent of multiparous labors were complicated by an active-phase abnormality. Friedman (1972) subdivided active-phase problems into protraction and arrest disorders. He defined protraction as a slow rate of cervical dilatation or descent, which for nulliparas was less than 1.2 cm dilatation per hour or less than 1 cm descent per hour. For multiparas, protraction was defined as less than 1.5 cm dilatation per hour or less than 2 cm descent per hour. He defined arrest as a complete cessation of dilatation or descent. Arrest of dilatation was defined as 2 hours with no cervical change, and arrest of descent as 1 hour without fetal descent.
The prognosis for protraction and arrest disorders differed considerably. Friedman found that approximately 30 percent of women with protraction disorders had cephalopelvic disproportion, compared with 45 percent of women in whom an arrest disorder developed. Factors contributing to both protraction and arrest disorders were excessive sedation, epidural analgesia, and fetal malposition. In both protraction and arrest disorders, Friedman recommended fetopelvic evaluation to identify cephalopelvic disproportion. Recommended therapy for protraction disorders was expectant management, whereas oxytocin was advised for arrest disorders in the absence of cephalopelvic disproportion.
Second Stage of Labor
Duration of Labor
Our understanding of the normal duration of labor may be clouded by the many clinical variables that affect conduct of labor in modern obstetrical units. Kilpatrick and Laros (1989) reported that the mean length of first- and second-stage labor was approximately 9 hours in nulliparous women without regional analgesia, and that the 95th percentile upper limit was 18.5 hours. Corresponding times for multiparous women were a mean of 6 hours with a 95th percentile maximum of 13.5 hours. These authors defined labor onset as the time when a woman recalled regular, painful contractions every 3 to 5 minutes that led to cervical change
Admission Procedures
+management of 1º stage
As discussed in Chapter 18, Electronic Fetal Monitoring, electronic fetal heart rate monitoring is routinely used for high-risk pregnancies commencing at admission. Some investigators recommend monitoring women with low-risk pregnancies upon admission as a test of fetal well-being—the so-called fetal admission test.
Most often, unless there has been bleeding in excess of bloody show, a vaginal examination is performed. The gloved index and second fingers are then introduced into the vagina while avoiding the anal region (Fig. 17-23). The number of vaginal examinations correlates with infection-related morbidity, especially in cases of early membrane ruptura.
The woman should be instructed during the antepartum period to be aware of fluid leakage from the vagina and to report such an event promptly. Rupture of the membranes is significant for three reasons. First, if the presenting part is not fixed in the pelvis, the possibility of umbilical cord prolapse and compression is greatly increased. Second, labor is likely to begin soon if the pregnancy is at or near term. Third, if delivery is delayed after membrane rupture, intrauterine infection is more likely as the time interval increases. A pH above 6.5 is consistent with ruptured membranes.
Cervical Examination
The degree of cervical effacement usually is expressed in terms of the length of the cervical canal compared with that of an uneffaced cervix. When the length of the cervix is reduced by one half, it is 50-percent effaced. When the cervix becomes as thin as the adjacent lower uterine segment, it is completely, or 100-percent, effaced
Cervical dilation
The position of the cervix is determined by the relationship of the cervical os to the fetal head and is categorized as posterior, midposition, or anterior. Along with position, the consistency of cervix is determined to be soft, firm, or intermediate between these two.
The level—or station—of the presenting fetal part in the birth canal is described in relationship to the ischial spines, which are halfway between the pelvic inlet and the pelvic outlet. When the lowermost portion of the presenting fetal part is at the level of the spines, it is designated as being at zero (0) station. In the past, the long axis of the birth canal above and below the ischial spines was arbitrarily divided into thirds by some and into fifths (approximately 1 cm) by other groups. In 1989, the American College of Obstetricians and Gynecologists adopted the classification of station that divides the pelvis above and below the spines into fifths. Each fifth represents a centimeter above or below the spines. Thus, as the presenting fetal part descends from the inlet toward the ischial spines, the designation is –5, –4, –3, –2, –1, then 0 station. Below the spines, as the presenting fetal part descends, it passes +1, +2, +3, +4, and +5 stations to delivery. Station +5 cm corresponds to the fetal head being visible at the introitus. If the leading part of the fetal head is at 0 station or below, most often the fetal head has engaged—thus, the biparietal plane has passed through the pelvic inlet. If the head is unusually molded or if there is an extensive caput formation or both, engagement might not have taken place although the head appears to be at 0 station.
When the woman is admitted in labor, most often the hematocrit or hemoglobin concentration should be rechecked. We obtain a urine specimen for protein determination in hypertensive women only.
Management of the First Stage of Labor
This is discussed in detail in Chapter 18. Briefly, the American Academy of Pediatrics and American College of Obstetricians and Gynecologists (2007) recommend that during the first stage of labor, in the absence of any abnormalities, the fetal heart rate should be checked immediately after a contraction at least every 30 minutes and then every 15 minutes during the second stage.
Uterine Contractions
Although usually assessed by electronic monitoring as also discussed in Chapter 18, contractions can be both quantitatively and qualitatively evaluated manually. With the palm of the hand resting lightly on the uterus, the time of contraction onset is determined. Its intensity is gauged from the degree of firmness the uterus achieves. At the acme of effective contractions, the finger or thumb cannot readily indent the uterus during a "firm" contraction. The time at which the contraction disappears is noted next. This sequence is repeated to evaluate the frequency, duration, and intensity of uterine contractions.
Food should be withheld during active labor and delivery
IV fluids??? Shrivastava and associates (2009) noted shorter labors in nulliparas delivering vaginal who were provided an intravenous normal saline (NS) with dextrose solution compared with those given NS solution only
Any maternal position
Bladder distension should be avoided because it can hinder descent of the fetal presenting part and lead to subsequent bladder hypotonia and infection. During each abdominal examination, the suprapubic region should be inspected and palpated to detect distension. If the bladder is readily seen or palpated above the symphysis, the woman should be encouraged to void. At times, she can ambulate with assistance to a toilet and successfully void, even though she cannot void on a bedpan. If the bladder is distended and she cannot void, catheterization is indicated
Management of the Second Stage of Labor
With full cervical dilatation, which signifies the onset of the second stage, a woman typically begins to bear down. With descent of the presenting part, she develops the urge to defecate. Uterine contractions and the accompanying expulsive forces may now last 11/2 minutes and recur at an interval no longer than 1 minute. As discussed in Second Stage of Labor, the median duration of the second stage is 50 minutes in nulliparas and 20 minutes in multiparas, although the interval can be highly variable
Expulsive Efforts
In most cases, bearing down is reflexive and spontaneous during second-stage labor. Occasionally, a woman may not employ her expulsive forces to good advantage and coaching is desirable. Her legs should be half-flexed so that she can push with them against the mattress. When the next uterine contraction begins, she is instructed to exert downward pressure as though she were straining at stool. In a randomized study from Istanbul, Yildirim and Beji (2008) reported that open-glottis pushing while breathing out was superior to the closed-glottis breath-held Valsalva-type pushing. The former method resulted in a shorter second stage and better cord acid-base values. A woman is not encouraged to push beyond the completion of each contraction. Instead, she and her fetus should be allowed to rest and recover. During this period of actively bearing down, the fetal heart rate auscultated immediately after the contraction is likely to be slow but should recover to normal range before the next expulsive effort
They found that the supported upright position had no advantages over the recumbent one. As the head descends through the pelvis, feces frequently are expelled by the woman. With further descent, the perineum begins to bulge and the overlying skin becomes stretched. Now the scalp of the fetus may be visible through the vulvar opening. At this time, the woman and her fetus are prepared for delivery.
Spontaneus delivery- delivery of the head. With each contraction, the perineum bulges increasingly. The vulvovaginal opening is dilated by the fetal head (Fig. 17-24), gradually forming an ovoid and, finally, an almost circular opening (Fig. 17-25). This encirclement of the largest head diameter by the vulvar ring is known as crowning. Unless an episiotomy has been made as described later, the perineum thins and especially in nulliparous women, may undergo spontaneous laceration. Slow delivery of the head while instructing the mother not to push may decrease lacerations according to Laine and co-workers (2008). The anus becomes greatly stretched and protuberant, and the anterior wall of the rectum may be easily seen through it.
Ritgen Maneuver
When the head distends the vulva and perineum enough to open the vaginal introitus to a diameter of 5 cm or more, a towel-draped, gloved hand may be used to exert forward pressure on the chin of the fetus through the perineum just in front of the coccyx. Concurrently, the other hand exerts pressure superiorly against the occiput (Fig. 17-26). This maneuver is simpler than that originally described by Ritgen (1855), and it is customarily designated the modified Ritgen maneuver
They preferred the "hands-poised" method, in which the attendant did not touch the perineum during delivery of the head. This method had similar associated laceration rates as the modified Ritgen maneuver, but with a lower incidence of third-degree tears.
Most often, the shoulders appear at the vulva just after external rotation and are born spontaneously. If delayed, immediate extraction may appear advisable. The sides of the head are grasped with two hands, and gentle downward traction is applied until the anterior shoulder appears under the pubic arch (Fig. 17-29). Some prefer to deliver the anterior shoulder prior to suctioning the nasopharynx or checking for a nuchal cord to avoid shoulder dystocia. Next, by an upward movement, the posterior shoulder is delivered. Hooking the fingers in the axillae should be avoided. This may injure the nerves of the upper extremity and produce a transient or possibly permanent paralysis. Traction, furthermore, should be exerted only in the direction of the long axis of the neonate. If applied obliquely, it causes bending of the neck and excessive stretching of the brachial plexus.
Clear the nasopharinx
Nuchal Cord
Following delivery of the anterior shoulder, a finger should be passed to the fetal neck to determine whether it is encircled by one or more coils of the umbilical cord (Fig. 17-30). A nuchal cord is found in approximately 25 percent of deliveries and ordinarily causes no harm. If a coil of umbilical cord is felt, it should be slipped over the head if loose enough. If applied too tightly, the loop should be cut between two clamps and the neonate promptly delivered
Clamping the Cord
The umbilical cord is cut between two clamps placed 4 to 5 cm from the fetal abdomen, and later an umbilical cord clamp is applied 2 to 3 cm from the fetal abdomen. A plastic clamp that is safe, efficient, and fairly inexpensive, such as the Double Grip Umbilical Clamp
Timing of Cord Clamping
If after delivery the newborn is placed at or below the level of the vaginal introitus for 3 minutes and the fetoplacental circulation is not immediately occluded by cord clamping, an average of 80 mL of blood may be shifted from the placenta to the neonate (Yao and Lind, 1974). This provides approximately 50 mg of iron, which reduces the frequency of iron-deficiency anemia later in infancy. At the same time, however, increased bilirubin from the added erythrocytes contributes further to hyperbilirubinemia (see Chap. 29, Hyperbilirubinemia). In their recent Cochrane Database review of randomized trials, McDonald and Middleton (2008) reported that delaying cord clamping until 1 minute after birth increased the newborn hemoglobin concentration 2.2 g/dL compared with clamping within the first 60 seconds. At the same time, early clamping reduced the risk of phototherapy by 40 percent.
Our policy is to clamp the cord after first thoroughly clearing the airway, all of which usually requires approximately 30 seconds. The newborn is not elevated above the introitus at vaginal delivery or much above the maternal abdominal wall at the time of cesarean delivery
Management of the Third Stage of Labor
Immediately after delivery of the newborn, the size of the uterine fundus and its consistency are examined. If the uterus remains firm and there is no unusual bleeding, watchful waiting until the placenta separates is the usual practice. Massage is not employed, but the fundus is frequently palpated to make certain that it does not become atonic and filled with blood from placental separation.
Because attempts to express the placenta prior to its separation are futile and possibly dangerous, the clinician should be alert to the following signs of placental separation:
1. The uterus becomes globular and as a rule, firmer
2. There is often a sudden gush of blood
3. The uterus rises in the abdomen because the placenta, having separated, passes down into the lower uterine segment and vagina. Here, its bulk pushes the uterus upward
4. The umbilical cord protrudes farther out of the vagina, indicating that the placenta has descended.
These signs sometimes appear within 1 minute after delivery of the newborn and usually within 5 minutes. When the placenta has separated, it should be determined that the uterus is firmly contracted. The mother may be asked to bear down, and the intra-abdominal pressure may be adequate to expel the placenta. If these efforts fail or if spontaneous expulsion is not possible because of anesthesia, then after ensuring that the uterus is contracted firmly, pressure is exerted with the hand on the fundus to propel the detached placenta into the vagina, as depicted and described in Figure 17-31. This approach has been termed physiological management as later contrasted with active management of the third stage .
Delivery of the Placenta
Expression of the placenta should never be forced before placental separation lest the uterus becomes inverted. Traction on the umbilical cord must not be used to pull the placenta out of the uterus. Uterine inversion is one of the grave complications associated with delivery, and it constitutes an emergency requiring immediate attention (see Chap. 35, Inversion of the Uterus). As downward pressure toward the vagina is applied to the body of the uterus, the umbilical cord is kept slightly taut (see Fig. 17-31). The uterus is then lifted cephalad with the abdominal hand. This maneuver is repeated until the placenta reaches the introitus (Prendiville and associates, 1988b). As the placenta passes through the introitus, pressure on the uterus is stopped. The placenta is then gently lifted away from the introitus (Fig. 17-32). Care is taken to prevent the membranes from being torn off and left behind. If the membranes start to tear, they are grasped with a clamp and removed by gentle teasing (Fig. 17-33). The maternal surface of the placenta should be examined carefully to ensure that no placental fragments are left in the uterus
Manual Removal of Placenta
Occasionally, the placenta will not separate promptly. This is especially common in cases of preterm delivery (Dombrowski and colleagues, 1995). If there is brisk bleeding and the placenta cannot be delivered by the above technique, manual removal of the placenta is indicated, using the safeguards described in Chapter 35, Technique of Manual Placental Removal. It is unclear as to the length of time that should elapse in the absence of bleeding before the placenta is manually removed (Deneux-Tharaux and co-workers, 2009). If induction analgesia is still intact, some obstetricians practice routine manual removal of any placenta that has not separated spontaneously by the time they have completed delivery of the newborn and care of the cord. Proof of the benefits of this practice, however, has not been established, and most obstetricians await spontaneous placental separation unless bleeding is excessive. The American College of Obstetricians and Gynecologists (2003b) has concluded that there are no data to either support or refute the use of prophylactic antimicrobials when manual removal is performed
Management of the Third Stage
Uterine massage following placental delivery is recommended by many to prevent postpartum hemorrhage. We support this but note that evidence for this practice is lacking (Hofmeyr and associates, 2008). Oxytocin, ergonovine, and methylergonovine are all employed widely in the normal third stage of labor, but the timing of their administration differs in various institutions. Oxytocin, and especially ergonovine, given before delivery of the placenta will decrease blood loss (Prendiville and associates, 1988a). If they are given before delivery of the placenta, however, they may entrap an undiagnosed, undelivered second twin. However, Jackson and colleagues (2001) randomly assigned 1486 women to infusions of 20 units of oxytocin diluted in 500 mL normal saline begun before or after placental delivery and found no differences in outcomes.
If an intravenous infusion is in place, our standard practice has been to add 20 units (2 mL) of oxytocin per liter of infusate. This solution is administered after delivery of the placenta at a rate of 10 mL/min (200 mU/min) for a few minutes until the uterus remains firmly contracted and bleeding is controlled. The infusion rate then is reduced to 1 to 2 mL/min until the mother is ready for transfer from the recovery suite to the postpartum unit. The infusion is usually then discontinued
Oxitocin: Mean pulse rate increased 28 bpm, mean arterial pressure decreased 33 mm Hg, and electrocardiogram changes of myocardial ischemia as well as chest pain and subjective discomfort were noted. These hemodynamic changes could be dangerous for women hypovolemic from hemorrhage or those with cardiac disease. Thus, oxytocin should not be given intravenously as a large bolus. Rather, it should be given as a dilute solution by continuous intravenous infusion or as an intramuscular injection in a dose of 10 USP units. In cases of postpartum hemorrhage, direct injection into the uterus, either transvaginally or transabdominally, following a vaginal birth or cesarean delivery has proven effective.
The use of nipple stimulation in the third stage of labor also has been shown to increase uterine pressures and to decrease third-stage duration and blood loss (Irons and associates, 1994). Indeed, results were similar to those achieved using the combination of oxytocin (5 units) and ergometrine (0.5 mg).
Prostaglandins
Analogs of prostaglandins are not used routinely for management of third-stage labor. Villar and colleagues (2002) reviewed prophylactic use of misoprostol to prevent postpartum hemorrhage and concluded that oxytocin or oxytocin-ergot preparations are more effective
First-degree lacerations involve the fourchette, perineal skin, and vaginal mucous membrane but not the underlying fascia and muscle (Fig. 17-34). These included periurethral lacerations, which may bleed profusely. Second-degree lacerations involve, in addition, the fascia and muscles of the perineal body but not the anal sphincter. These tears usually extend upward on one or both sides of the vagina, forming an irregular triangular injury. Third-degree lacerations extend farther to involve the anal sphincter. A fourth-degree laceration extends through the rectum's mucosa to expose its lumen.
Perineal tears may follow any vaginal delivery, but Combs and associates (1990) identified factors associated with an increased risk of third- and fourth-degree lacerations. These include midline episiotomy, nulliparity, second-stage arrest of labor, persistent occiput posterior position, mid or low forceps, use of local anesthetics, and Asian race.
