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.
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