CHAPTER 23: Abnormal Labor

The term dystocia as described by Williams in the first edition of this text still applies today. It literally means difficult labor and is characterized by abnormally slow labor progress. Similar to the factors described by Williams, dystocia arises from three distinct abnormality categories. First, uterine contractions may be insufficiently strong or inappropriately coordinated to efface and dilate the cervix—uterine dysfunction. Also, voluntary maternal muscle effort during second-stage labor may be inadequate. Second, fetal abnormalities of presentation, position, or anatomy may slow progress. Last, structural changes can contract the maternal bony pelvis. Or, soft tissue abnormalities of the reproductive tract may form an obstacle to fetal descent. More simply, these alterations can be mechanistically simplified into three categories that include abnormalities of the powers—uterine contractility and maternal expulsive effort; of the passenger—the fetus; and of the passage—the pelvis and lower reproductive tract.

Descriptors

Abnormalities shown in Table 23-1 often interact singly or in combination to produce dysfunctional labor. Commonly used expressions today such as cephalopelvic disproportion and failure to progress are used to describe ineffective labors. Of these, cephalopelvic disproportion is a term that came into use before the 20th century to describe obstructed labor resulting from disparity between the fetal head size and maternal pelvis. But, the term originated at a time when the main indication for cesarean delivery was overt pelvic contracture due to rickets (Olah, 1994). Such absolute disproportion is now rare, and most cases result from malposition of the fetal head within the pelvis (asynclitism) or from ineffective uterine contractions. True disproportion is a tenuous diagnosis because many women who undergo cesarean delivery for this reason subsequently deliver even larger newborns vaginally in subsequent pregnancies. A second phrase, failure to progress in either spontaneous or stimulated labor, has become an increasingly popular description of ineffectual labor. This term reflects lack of progressive cervical dilation or lack of fetal descent. Neither of these two terms is specific.

Mechanisms of Dystocia

At the end of pregnancy, the fetal head encounters a relatively thick lower uterine segment and undilated cervix. With the onset of labor, the factors influencing progress are uterine contractions, cervical resistance, and the forward pressure exerted by the leading fetal part.

After complete cervical dilation, the mechanical relationship between the fetal head size and position and the pelvic capacity, namely fetopelvic proportion, becomes clearer as the fetus attempts to descend. Because of this, abnormalities in fetopelvic proportions become more apparent once the second stage is reached.

Uterine muscle malfunction can result from uterine overdistention, obstructed labor, or both. Thus, ineffective labor is generally accepted as a possible warning sign of fetopelvic disproportion. Although artificial separation of labor abnormalities into pure uterine dysfunction and fetopelvic disproportion simplifies classification, it is an incomplete characterization because these two abnormalities are closely interlinked. Indeed, the bony pelvis rarely limits vaginal delivery. In the absence of objective means of precisely distinguishing these two causes of labor failure, clinicians must rely on a trial of labor to determine if labor can be successful in effecting vaginal delivery.

Cervical dilation as well as propulsion and expulsion of the fetus are brought about by uterine contractions. During second-stage labor, these contractions are then reinforced by voluntary or involuntary muscular action of the abdominal wall—“pushing.” The diagnosis of uterine dysfunction in the latent phase is difficult and sometimes can be made only in retrospect. Women who are not yet in active labor commonly are erroneously treated for this dysfunction.

Beginning in the 1960s, at least three significant advances have aided treatment of uterine dysfunction. First is the realization that undue labor prolongation may contribute to maternal and perinatal morbidity and mortality rates. Second, dilute intravenous infusion of oxytocin is administered to treat certain types of uterine dysfunction. Last, cesarean delivery is selected rather than difficult midforceps delivery when oxytocin fails or its use is inappropriate.

Types of Uterine Dysfunction

Reynolds and coworkers (1948) emphasized that uterine contractions of normal labor are characterized by a gradient of myometrial activity. These forces are greatest and last longest at the fundus—considered fundal dominance—and they diminish toward the cervix. Caldeyro-Barcia and colleagues (1950) from Montevideo, Uruguay, inserted small balloons into the myometrium at various levels (Chap. 24, Intrapartum Surveillance of Uterine Activity). They reported that in addition to a gradient of activity, the onset of the contractions differed in the fundus, midzone, and lower uterine segments. Larks (1960) described the stimulus as starting in one cornu and then several milliseconds later in the other. The excitation waves then join and sweep over the fundus and down the uterus. Normal spontaneous contractions often exert pressures approximating 60 mm Hg (Hendricks, 1959). Even so, the Montevideo group ascertained that the lower limit of contraction pressure required to dilate the cervix is 15 mm Hg.

From these observations, two physiological types of uterine dysfunction are defined. In the more common hypotonic uterine dysfunction, there is no basal hypertonus, and uterine contractions have a normal gradient pattern (synchronous). However, pressure during a contraction is insufficient to dilate the cervix.

In the second type, hypertonic uterine dysfunction or incoordinate uterine dysfunction, either basal tone is elevated appreciably or the pressure gradient is distorted. Gradient distortion may result from more forceful contraction of the uterine midsegment than the fundus, from complete asynchrony of the impulses originating in each cornu, or from a combination of these two.

Labor Disorders

Latent-Phase Prolongation

These just-described types of uterine dysfunction can in turn lead to labor abnormalities (Table 23-2). First, the latent phase may be prolonged, which is defined as exceeding 20 hours in the nullipara and 14 hours in the multipara. In some, uterine contractions cease, suggesting false labor. In the remainder, an abnormally long latent phase persists and is often treated with oxytocin stimulation.

Active-Phase Disorders

In active labor, disorders are divided into slower-than-normal progress—a protraction disorder—or complete cessation of progress—an arrest disorder. Terms presented in Table 23-2 and their diagnostic criteria more precisely describe abnormal labor. To be diagnosed with either of these, a woman must be in the active phase of labor, which is defined by cervical change.

These criteria and management of abnormal labor have recently undergone a sea change. In 2014, the American College of Obstetricians and Gynecologists and the Society for Maternal–Fetal Medicine issued their first Obstetric Care Consensus titled Safe Prevention of the Primary Cesarean Delivery. It was reaffirmed in 2016. This consensus statement was a response to concerns that cesarean delivery was overused in the United States. Namely, approximately one in three women who give birth each year undergoes this surgery (Fig. 31-1). New recommendations from the Consensus Committee are based on “more recent data used to revise the definition of contemporary normal labor progress” and reflect a significant revision of the preexisting understanding of abnormal labor.

Active-Phase Protraction

Of active-phase disorders, protraction disorders are less well described, and the time necessary before diagnosing slow progress is undefined. The World Health Organization (1994) has proposed a labor management partograph in which protraction is defined as <1 cm/hr cervical dilation for a minimum of 4 hours. These criteria were adapted from those of Cohen and Friedman (1983) and shown in Table 23-2. For this disorder, observation for further progress is appropriate treatment. If insufficient Montevideo units are noted, oxytocin augmentation is initiated. According to the Consensus Committee (2016), slow but progressive first-stage labor should not be an indication for cesarean delivery.

Active-Phase Arrest

Handa and Laros (1993) diagnosed active-phase arrest, defined as no dilation for 2 hours or more, in 5 percent of term nulliparas. This incidence has not changed since the 1950s (Friedman, 1978). Inadequate uterine contractions, defined as less than 180 Montevideo units, calculated as shown in Figure 23-1, were diagnosed in 80 percent of women with active-phase arrest. Hauth and coworkers (1986, 1991) reported that when labor is effectively induced or augmented with oxytocin, 90 percent of women achieve 200 to 225 Montevideo units, and 40 percent achieve at least 300 Montevideo units. These results suggest that certain minimums of uterine activity should be achieved before performing cesarean delivery for dystocia.

Other criteria should also be met. First, the latent phase should be completed, and the cervix is dilated ≥4 cm. Also, a uterine contraction pattern of 200 Montevideo units or more in a 10-minute period has been present for 2 or more hours without cervical change. Rouse and associates (1999) challenged the “2-hour rule” on the grounds that a longer time, that is, at least 4 hours, is necessary before concluding that the active phase of labor has failed. We agree. The Consensus Committee (2016) has expanded the criteria, as described next.

Obstetric Care Consensus Committee

There are four recommendations by the Consensus Committee (2016) applicable to management of the first-stage labor. The first admonishes against cesarean delivery in the latent phase of labor. Specifically, a prolonged latent phase is not an indication for cesarean delivery. This guideline is not new and is traceable to the work of Friedman (1954), on which traditional tenets are based.

The second directive, too, is conventional practice. It recommends against cesarean delivery if labor is progressive but slow—a protraction disorder. This instance is typically managed with observation, assessment of uterine activity, and stimulation of contractions as needed.

A third instruction addresses the cervical dilation threshold that serves to herald active labor. Namely, a cervical dilation of 6 cm—not 4 cm—is now the recommended threshold. Thus, before this threshold, standards for active-phase progress should not be applied.

A fourth stipulation notes that cesarean delivery for active-phase arrest “should be reserved for women at or beyond 6 cm of dilation with ruptured membranes who fail to progress despite 4 hours of adequate uterine activity, or at least 6 hours of oxytocin administration with inadequate contractions and no cervical change.”

According to the Consensus Committee (2016), the “6-cm rule” stems from labor data from the Consortium on Safe Labor study (Zhang, 2010). This study derived its numbers from a retrospective observational dataset built from abstracted labor and delivery information from 19 hospitals across the United States. Various statistical methods and heavy manipulations of these numbers were used (Cohen, 2015b). As shown in Figure 23-2, a total of 62,415 women were analyzed after excluding all women with cesarean deliveries or asphyxiated newborns. The Consensus Committee (2016) was explicit that “the Consortium on Safe Labor data, rather than the standards proposed by Friedman should inform evidence-based labor management.” Described in Chapter 21 (First Stage of Labor), the latter are labor curves that have been used since first proposed by Friedman (1955).

Critics of the Consensus Committee (2016) recommendations note that the Consortium on Safe Labor data were derived from clinical settings with a net cesarean rate of 30 percent. Thus, adherence to the new recommendations may fail to achieve desired cesarean rate reductions. Also, the study lacked a focus on neonatal safety, given that all the asphyxiated neonates were excluded. Supporters note that the study of prolonged first-stage labors by Cheng and coworkers (2010) found higher rates of cesarean delivery and chorioamnionitis but not higher rates of neonatal morbidity. However, Harper and associates (2014) analyzed maternal and neonatal adverse outcomes related to first-stage labor lengths. In 5030 women, first-stage labor durations were divided into those <90th percentile or those ≥90th percentile, with incremental increases thereafter. These authors concluded that a longer first-stage labor was associated with maternal and neonatal complications and that these should be balanced against the risks of cesarean delivery. This concern for adverse fetal and maternal effects resulting from the new Consensus Committee guidelines was echoed by Cohen and Friedman (2015a,b).

Another caveat notes that the efficacy of these recommendations to achieve their primary goal is limited. In support, one retrospective cohort study of 200 women undergoing induction or augmentation before and 200 after guideline changes found a cesarean delivery rate drop from 35 to 25 percent (Wilson-Leedy, 2016). However, although the study was not powered to assess adverse neonatal outcomes, nonsignificant but higher rates of umbilical artery gases with pH <7 and base deficit >12 mmol/L were noted in the postguideline group (Marte, 2016). In another evaluation of outcomes before and after guideline implementation, cesarean delivery rates were unchanged. Overall, there were no associations between length of arrest and maternal or neonatal morbidity, but neonatal respiratory morbidity rates rose in newborns of women with longer periods of arrested dilation (Rosenbloom, 2017). Thus, additional studies are needed to define risks and benefits of the new guidelines.

Background for the 6-cm Rule

Review of selected publications helps explain the Consensus Committee (2016) evolution to a 6-cm rule. First, Zhang and coworkers (2002) compared study populations from one report by Friedman (1955) with one from 1992 to 1996 at Tripler Army Hospital, Hawaii (Table 23-3). The Friedman labor curves reflected women in spontaneous labor with infrequent use of neuraxial labor analgesia or oxytocin augmentation. In contrast, in the Tripler cohort, approximately 50 percent of women had neuraxial analgesia or augmentation. The rate of cervical change for the ensuing hour at each cervical dilation between 2 and 9 cm in the Tripler group is shown in Table 23-4. Progress is slow between 4 and 6 cm but accelerates thereafter. This could reasonably be interpreted as the active phase beginning at 6 cm. Shown in Figure 23-3 are the labor curves for Friedman (1955) compared to the Tripler cohort. These differ in a flattening of the active phase beginning at 3 to 4 cm in the Tripler group. This is consistent with the labor results obtained in the Safe Labor Consortium study (Zhang, 2010). Namely, the 6-cm rule for active labor derives from a slowing of the rate or flattening of the slope of cervical change in first-stage labor.

Neuraxial analgesia delays the active phase of spontaneous labor and flattens the slope. For example, Gambling and associates (1998) compared combined spinal epidural (CSE) labor analgesia against intermittent intravenous (IV) boluses of 50 mg meperidine in 1223 nulliparas in spontaneous labor at term. The active phase of labor was diagnosed when cervical dilation was 4 cm in the presence of regular uterine contractions. As shown in Table 23-5, mean cervical dilation at first pain relief was 5 cm in both study groups. Oxytocin augmentation rates were significantly greater in the CSE group. Also, the first analgesia-to-delivery interval was lengthened (5 versus 4 hours). However, cesarean delivery rates did not differ significantly.

Importantly, the effect of neuraxial analgesia to slow active-phase labor should not negatively affect use of neuraxial pain relief. As described in Chapter 25 (Effects on Labor), investigators at Parkland Hospital completed five randomized trials involving a total of 2703 nulliparas in spontaneous labor at term (Sharma, 2004). Various neuraxial techniques were compared with meperidine for pain relief. First-stage labor was significantly lengthened with neuraxial analgesia (8.1 versus 6.6 hours) but cesarean delivery rates for dystocia were unaffected (9.1 versus 8.1 percent).

In sum, neuraxial analgesia slows the active phase of first-stage labor. This currently is empirically corrected by augmentation of uterine contractions. Thus, recommendations of the Consensus Committee regarding protraction disorders are correct. Second, the Consensus Committee (2016) suggests that cesarean deliveries for dystocia are being done before 6 cm cervical dilation. However, there is good reason to believe that all of the Committee’s first-stage recommendations are actually already empirically in use but are concurrent with an overall cesarean delivery rate in excess of 30 percent. For example, Table 23-6 lists labor characteristics in 9000 women with primary cesarean deliveries for dystocia at 13 university hospitals between 1999 and 2000 (Alexander, 2003). Notably, the median cervical dilation at the time of cesarean for dystocia was 6 cm. Moreover, Figure 23-4 depicts primary cesarean delivery rates for dystocia at Parkland Hospital between 1988 and 2017. The rate has not changed significantly in 28 years. Thus, Consensus Committee (2016) guidelines may fail to prevent additional cesareans for dystocia. Again, additional studies are needed.

Second-Stage Descent Disorders

Fetal descent largely follows complete dilation. Moreover, the second stage incorporates many of the cardinal movements necessary for the fetus to negotiate the birth canal (Chap. 22, Descent). Accordingly, disproportion of the fetus and pelvis frequently becomes apparent during second-stage labor.

Similar to first-stage labor, time boundaries have been supported to limit second-stage duration to minimize adverse maternal and fetal outcomes. The second stage in nulliparas is limited to 2 hours and extended to 3 hours when regional analgesia is used. For multiparas, 1 hour is the limit, extended to 2 hours with regional analgesia.

Cohen (1977) investigated the fetal effects of second-stage labor length at Beth Israel Hospital. He included 4403 term nulliparas in whom electronic fetal heart rate monitoring was performed. The neonatal mortality rate was not increased in women whose second-stage labor exceeded 2 hours. Epidural analgesia was used commonly, and this likely accounted for the large number of pregnancies with a prolonged second stage. These data influenced decisions to permit an additional hour for the second stage when regional analgesia is used.

Menticoglou and coworkers (1995a,b) also challenged the prevailing dicta on second-stage duration. Their concern stemmed from grave neonatal injuries associated with forceps rotations to shorten second-stage labor. As a result, they allowed a longer second stage to decrease the operative vaginal delivery rate. Between 1988 and 1992, second-stage labor exceeded 2 hours in a fourth of 6041 nulliparas at term. Labor epidural analgesia was used in 55 percent. The length of the second stage, even in those lasting up to 6 hours or more, was not related to neonatal outcome. These results were attributed to careful use of electronic monitoring and scalp pH measurements. These investigators concluded that there is no compelling reason to intervene with a possibly difficult forceps or vacuum extraction because a certain number of hours have elapsed. They observed, however, that after 3 hours in the second stage, delivery by cesarean or other operative method increased progressively. By 5 hours, the prospects for spontaneous delivery in the subsequent hour were only 10 to 15 percent.

Newer guidelines have been promoted by the Consensus Committee (2016) for second-stage labor. These recommend allowing a nullipara to push for at least 3 hours and a multipara to push for at least 2 hours before second-stage labor arrest is diagnosed. One caveat is that maternal and fetal status should be reassuring. These authors provide options to these times before cesarean delivery is performed. Namely, longer durations may be appropriate as long as progress is documented. Also, a specific maximal length of time spent in second-stage labor beyond which all women should undergo operative delivery has not been identified.

Intuitively, the goal to lower cesarean delivery rates is best balanced with one to ensure neonatal safety. And, it is problematic that no robust data on neonatal outcomes support the safety of allowing prolonged second-stage labor. Data from many evaluations reveal that serious newborn consequences attend second-stage labors longer than 3 hours (Allen, 2009; Bleich, 2012; Laughon, 2014; Leveno, 2016; Rosenbloom, 2017). Other data, when adjusted for labor variables, show no difference in neonatal complications for these longer second stages (Cheng, 2004; Le Ray, 2009; Rouse, 2009). Grobman and colleagues (2016) have argued that the absolute number of such adverse outcomes is small and “overall outcomes remain good.” That said, some of the complications are severe. Thus, to fully ascertain specific effect of these guidelines on morbidity rates, randomized controlled trials are needed.

It is possible that prolonged first-stage labor presages that with the second stage. Nelson and associates (2013) studied the relationships between the lengths of the first and second stages of labor in 12,523 nulliparas at term delivered at Parkland Hospital. The second stage significantly lengthened concomitantly with increasing first-stage duration. The 95th percentile was 15.6 and 2.9 hours for the first and second stages, respectively. Women with first stages lasting longer than 15.6 hours (>95th percentile) had a 16-percent rate of second-stage labor lasting 3 hours (95th percentile). This compared with a 4.5-percent rate of prolonged second stages in women with first-stage labors lasting <95th percentile.

Maternal Pushing Efforts

With full cervical dilation, most women cannot resist the urge to “bear down” or “push” each time the uterus contracts (Chap. 22, Labor Management Protocols). The combined force created by contractions of the uterus and abdominal musculature propels the fetus downward. At times, force created by abdominal musculature is compromised sufficiently to slow or even prevent spontaneous vaginal delivery. Heavy sedation or regional analgesia may reduce the reflex urge to push and may impair the ability to contract abdominal muscles effectively. In other instances, the inherent urge to push is overridden by the intense pain created by bearing down. Two approaches to second-stage pushing in women with epidural analgesia have yielded contradictory results. The first advocates pushing forcefully with contractions after complete dilation, regardless of the urge to push. With the second, analgesia infusion is stopped and pushing begins only after the woman regains the sensory urge to bear down. Fraser and coworkers (2000) found that delayed pushing reduced difficult operative deliveries, whereas Manyonda and associates (1990) reported the opposite. Hansen and colleagues (2002) randomly assigned 252 women with epidural analgesia to one of the two approaches. No adverse maternal or neonatal outcomes were linked to delayed pushing despite significantly prolonging second-stage labor. Plunkett and coworkers (2003), in a similar study, confirmed these findings.

Fetal Station at Labor Onset

Descent of the leading edge of the presenting part to the level of the ischial spines (0 station) is defined as engagement. A higher station at the onset of labor is significantly linked with subsequent dystocia (Friedman, 1965, 1976; Handa, 1993). Roshanfekr and associates (1999) analyzed fetal station in 803 nulliparas at term in active labor. At admission, the third with the fetal head at or below 0 station had a 5-percent cesarean delivery rate. This compared with a 14-percent rate for those with higher stations. The prognosis for dystocia, however, was not related to incrementally higher fetal head stations above the pelvic midplane (0 station). Importantly, 86 percent of nulliparous women without fetal head engagement at diagnosis of active labor delivered vaginally. These observations apply especially for parous women because the head typically descends later in labor.

Risks for Uterine Dysfunction

Various labor factors have been implicated as causes of uterine dysfunction. As described, neuraxial analgesia can slow labor and has been associated with lengthening both first and second stages of labor and slowing the rate of fetal descent.

Chorioamnionitis is associated with prolonged labor, and some clinicians have suggested that this maternal intrapartum infection itself contributes to abnormal uterine activity. Satin and coworkers (1992) studied the effects of chorioamnionitis on oxytocin stimulation in 266 pregnancies. Infection diagnosed late in labor was found to be a marker of cesarean delivery performed for dystocia. Specifically, 40 percent of women developing chorioamnionitis after requiring oxytocin for dysfunctional labor later required cesarean delivery for dystocia. However, this was not a marker in women diagnosed as having chorioamnionitis early in labor. It is likely that uterine infection in this clinical setting is a consequence of dysfunctional, prolonged labor rather than a cause of dystocia.

Membrane rupture at term without spontaneous uterine contractions complicates approximately 8 percent of pregnancies. In the past, labor stimulation was initiated if contractions did not begin after 6 to 12 hours. Practice-changing research included that of Hannah (1996) and Peleg (1999) and their associates, who enrolled a total of 5042 pregnancies with ruptured membranes in a randomized investigation. They measured the effects of induction versus expectant management and also compared induction using intravenous oxytocin with that using prostaglandin E₂ gel. There were approximately 1200 pregnancies in each of the four study arms. They concluded that labor induction with intravenous oxytocin was preferred management. This was based on significantly fewer intrapartum and postpartum infections in women whose labor was induced. There were no significant differences in cesarean delivery rates. Subsequent analysis by Hannah and coworkers (2000) indicated higher rates of adverse outcomes when expectant management at home was compared with in-hospital observation. Mozurkewich and associates (2009) reported lower rates of chorioamnionitis, metritis, and neonatal intensive care unit admissions for women with term ruptured membranes whose labors were induced compared with those managed expectantly. At Parkland Hospital, labor is induced soon after admission when ruptured membranes are confirmed at term. In those with hypotonic contractions or with advanced cervical dilation, oxytocin is selected to lower potential hyperstimulation risk. In those with an unfavorable cervix and no or few contraction, prostaglandin E₁ (misoprostol) is chosen to promote cervical ripening and contractions. The benefit of prophylactic antibiotics in women with ruptured membranes before labor at term is unclear (Passos, 2012). However, in those with membranes ruptured longer than 18 hours, antibiotics are instituted for group B streptococcal infection prophylaxis (Chap. 64, GBS Vaccine).

Labor can be too slow, but it also can be abnormally rapid. Precipitous labor and delivery is extremely rapid labor and delivery. It may result from an abnormally low resistance of the soft parts of the birth canal, from abnormally strong uterine and abdominal contractions, or rarely from the absence of painful sensations and thus a lack of awareness of vigorous labor.

Precipitous labor terminates in expulsion of the fetus in less than 3 hours. Using this definition, 25,260 live births—3 percent—were complicated by precipitous labor in the United States in 2013 (Martin, 2015). Despite this incidence, little published information describes maternal and perinatal outcomes.

For the mother, precipitous labor and delivery seldom are accompanied by serious maternal complications if the cervix is effaced appreciably and compliant, if the vagina has been stretched previously, and if the perineum is relaxed. Conversely, vigorous uterine contractions combined with a long, firm cervix and a noncompliant birth canal may lead to uterine rupture or extensive lacerations of the cervix, vagina, vulva, or perineum (Sheiner, 2004). It is in these latter circumstances that amnionic-fluid embolism most likely develops (Chap. 41, General Management). Precipitous labor is frequently followed by uterine atony. The uterus that contracts with unusual vigor before delivery is likely to be hypotonic after delivery. In one report of 99 term pregnancies, short labors were more common in multiparas who typically had contractions at intervals less than 2 minutes. Precipitous labors have been linked to cocaine abuse and associated with placental abruption, meconium, postpartum hemorrhage, and low Apgar scores (Mahon, 1994).

For the neonate, adverse perinatal outcomes from rapid labor may be increased considerably for several reasons. The tumultuous uterine contractions, often with negligible intervals of relaxation, prevent appropriate uterine blood flow and fetal oxygenation. Resistance of the birth canal may rarely cause intracranial trauma. Acker and coworkers (1988) reported that Erb or Duchenne brachial palsy was associated with such labors in a third of cases. Finally, during an unattended birth, the newborn may fall to the floor and be injured, or it may need resuscitation that is not immediately available.

As treatment, analgesia is unlikely to modify these unusually forceful contractions to a significant degree. The use of tocolytic agents such as magnesium sulfate or terbutaline is unproven in these circumstances. Use of general anesthesia with agents that impair uterine contractibility such as isoflurane is often excessively heroic. Certainly, any oxytocin being administered should be stopped immediately.

Pelvic Capacity

Fetopelvic disproportion arises from diminished pelvic capacity, from abnormal fetal size or presentation, or more usually from both. The pelvic inlet, midpelvis, or pelvic outlet may be contracted solely or in combination. Any contraction of the pelvic diameters that diminishes pelvic capacity can create dystocia during labor. Normal pelvic dimensions are additionally discussed and illustrated in Chapter 2 (Planes and Diameters of the Pelvis).

Contracted Inlet

Using clinical measures, it is important to identify the shortest anteroposterior diameter through which the fetal head must pass. Before labor, the fetal biparietal diameter averages from 9.5 to as much as 9.8 cm. Therefore, it might prove difficult or even impossible for some fetuses to pass through a pelvic inlet that has an anteroposterior diameter <10 cm. Mengert (1948) and Kaltreider (1952), employing x-ray pelvimetry, demonstrated that the incidence of difficult deliveries rises when either the anteroposterior diameter of the inlet is <10 cm or the transverse diameter is <12 cm. As expected, when both diameters are contracted, dystocia rates are much greater than when only one is contracted. Either of these measures is used to consider a pelvis contracted.

The anteroposterior diameter of the inlet, which is the obstetrical conjugate, is commonly approximated by manually measuring the diagonal conjugate, which is approximately 1.5 cm greater. Ascertainment of these measures is described in Chapter 2 (Planes and Diameters of the Pelvis). Therefore, inlet contraction usually is defined as a diagonal conjugate <11.5 cm.

A small woman is likely to have a small pelvis, but she is also likely to have a small neonate. Thoms (1937) studied 362 nulliparas and found that the mean birthweight of their offspring was significantly lower—280 g—in women with a small pelvis than in those with a medium or large pelvis.

Normally, cervical dilation is aided by hydrostatic action of the unruptured membranes or after their rupture, by direct application of the presenting part against the cervix. In contracted pelves, however, because the head is arrested in the pelvic inlet, the entire force exerted by the uterus acts directly on the portion of membranes that contact the dilating cervix. Consequently, early spontaneous rupture of the membranes is more likely.

After membrane rupture, absent pressure by the head against the cervix and lower uterine segment predisposes to less effective contractions. Hence, further dilation may proceed very slowly or not at all. Cibils and Hendricks (1965) reported that the mechanical adaptation of the fetal passenger to the bony passage plays an important part in determining the efficiency of contractions. The better the adaptation, the more efficient the contractions. Thus, cervical response to labor provides a prognostic view of labor outcome in women with inlet contraction.

A contracted inlet also plays an important part in the production of abnormal presentations. In nulliparas with normal pelvic capacity, the presenting part at term commonly descends into the pelvic cavity before labor onset. When the inlet is contracted considerably or there is marked asynclitism, descent usually does not take place until after labor onset, if at all. Cephalic presentations still predominate, but the head floats freely over the pelvic inlet or rests more laterally in one of the iliac fossae. Accordingly, very slight influences may cause the fetus to assume other presentations. In women with contracted pelves, face and shoulder presentations are encountered three times more frequently, and the cord prolapses four to six times more often.

Contracted Midpelvis

This finding is more common than inlet contraction. It frequently causes transverse arrest of the fetal head, which potentially can lead to a difficult midforceps operation or to cesarean delivery.

The obstetrical plane of the midpelvis extends from the inferior margin of the symphysis pubis through the ischial spines and touches the sacrum near the junction of the fourth and fifth vertebrae. A transverse line theoretically connecting the ischial spines divides the midpelvis into anterior and posterior portions (Fig. 2-16). The former is bounded anteriorly by the lower border of the symphysis pubis and laterally by the ischiopubic rami. The posterior portion is bounded dorsally by the sacrum and laterally by the sacrospinous ligaments, forming the lower limits of the sacrosciatic notch.

Average midpelvis measurements are as follows: transverse, or interischial spinous, 10.5 cm; anteroposterior, from the lower border of the symphysis pubis to the junction of S_4–5, 11.5 cm; and posterior sagittal, from the midpoint of the interspinous line to the same point on the sacrum, 5 cm. The definition of midpelvic contractions has not been established with the same precision possible for inlet contractions. Even so, the midpelvis is likely contracted when the sum of the interspinous and posterior sagittal diameters of the midpelvis—normally, 10.5 plus 5 cm, or 15.5 cm—falls to 13.5 cm or less. This concept was emphasized by Chen and Huang (1982) in evaluating possible midpelvic contraction. Midpelvic contraction is suspected whenever the interspinous diameter is <10 cm. When it measures <8 cm, the midpelvis is contracted.

Although no precise manual method permits measure of midpelvic dimensions, a suggestion of contraction sometimes can be inferred if the spines are prominent, the pelvic sidewalls converge, or the sacrosciatic notch is narrow. Moreover, Eller and Mengert (1947) noted that the relationship between the intertuberous and interspinous diameters of the ischium is sufficiently constant that narrowing of the interspinous diameter can be anticipated when the intertuberous diameter is narrow. A normal intertuberous diameter, however, does not always exclude a narrow interspinous diameter.

Contracted Outlet

This finding usually is defined as an interischial tuberous diameter of 8 cm or less. The pelvic outlet may be roughly likened to two triangles, with the interischial tuberous diameter constituting the base of both. The sides of the anterior triangle are the pubic rami, and its apex is the inferoposterior surface of the symphysis pubis. The posterior triangle has no bony sides but is limited at its apex by the tip of the last sacral vertebra—not the tip of the coccyx. Diminution of the intertuberous diameter with consequent narrowing of the anterior triangle must inevitably force the fetal head posteriorly. Floberg and associates (1987) reported that outlet contractions were found in almost 1 percent of more than 1400 unselected nulliparas with term pregnancies. A contracted outlet may cause dystocia not so much by itself but by an often-associated midpelvic contraction. Outlet contraction without concomitant midplane contraction is rare.

Although the disproportion between the fetal head and the pelvic outlet is not sufficiently great to give rise to severe dystocia, it may play an important part in perineal tears. With increased narrowing of the pubic arch, the occiput cannot emerge directly beneath the symphysis pubis but is forced farther down upon the ischiopubic rami. The perineum, consequently, becomes increasingly distended and thus exposed to risk of laceration.

Pelvic Fractures

Vallier (2012) reviewed experiences with pelvic fractures and pregnancy. Trauma from automobile collisions was the most common cause. Moreover, they note that fracture pattern, minor malalignment, and retained hardware are not absolute indications for cesarean delivery. In determining suitability for vaginal delivery, fracture healing requires 8 to 12 weeks and thus recent fracture merits cesarean delivery (Amorosa, 2013). A history of pelvic fracture warrants careful review of previous radiographs and possible imaging pelvimetry later in pregnancy.

Pelvic Capacity Estimation

The techniques for clinical evaluation using digital examination of the bony pelvis during labor are described in detail in Chapter 2 (Planes and Diameters of the Pelvis). The value of radiological imaging to assess pelvic capacity has also been examined. First, with x-ray pelvimetry alone, the prognosis for successful vaginal delivery in any given pregnancy with cephalic presentation cannot be established (Mengert, 1948). Similarly, one systematic review found insufficient evidence to support the use of x-ray pelvimetry with cephalic presentations (Pattinson, 2017).

Advantages of pelvimetry with computed tomography (CT) compared with those of conventional x-ray pelvimetry include greater accuracy and easier performance. With either method, costs are comparable, and x-ray exposure is small (Chap. 46, X-Ray Dosimetry). Depending on the machine and technique employed, fetal doses with CT pelvimetry may range from 250 to 1500 mrad (Moore, 1989).

Advantages of magnetic resonance (MR) pelvimetry include lack of ionizing radiation, accurate measurements, complete fetal imaging, and the potential for evaluating soft tissue dystocia (McCarthy, 1986; Stark, 1985). Zaretsky and colleagues (2005) used MR imaging to measure pelvic and fetal head volume to identify those women at greatest risk of undergoing cesarean delivery for dystocia. Significant associations were found between some of the measures and cesarean delivery for dystocia. However, these researchers could not with accuracy predict which individual woman would require cesarean delivery. Others have reported similar findings (Sporri, 1997).

Fetal Body and Head Size

Fetal size alone is seldom a suitable explanation for failed labor. Even with current technology, a fetal size threshold to predict fetopelvic disproportion is still elusive. Most cases of disproportion arise in fetuses whose weight is well within the range of the general obstetrical population. As shown in Figure 23-5, two thirds of neonates who required cesarean delivery after failed forceps delivery weighed <3700 g. Thus, other factors—for example, malposition of the head—obstruct fetal passage through the birth canal. These include asynclitism, occiput posterior position, and face or brow presentation.

For fetal head size estimation, clinical and radiographical methods to predict fetopelvic disproportion have proved disappointing. Mueller (1885) and Hillis (1930) described a clinical maneuver to predict disproportion. The fetal brow and the suboccipital region are grasped through the abdominal wall with the fingers, and firm pressure is directed downward in the axis of the inlet. If no disproportion exists, the head readily enters the pelvis, and vaginal delivery can be predicted. Thorp and coworkers (1993) performed a prospective evaluation of this Mueller-Hillis maneuver. They found no relationship between failed descent during the maneuver and subsequent labor dystocia.

Measurements of fetal head diameters using plain radiographical techniques are not used because of parallax distortions. The biparietal diameter and head circumference can be measured sonographically, and investigators have attempted to use this information in the management of dystocia. Thurnau and colleagues (1991) used the fetal-pelvic index to identify labor complications. Unfortunately, the sensitivity of such measurements to predict cephalopelvic disproportion is poor (Ferguson, 1998; Korhonen, 2015). We believe that no current method of measurement satisfactorily predicts fetopelvic disproportion based on head size.

Face Presentation

With this presentation, the neck is hyperextended so that the occiput is in contact with the fetal back, and the chin (mentum) is presenting (Fig. 23-6). The fetal face may present with the chin (mentum) anteriorly or posteriorly, relative to the maternal symphysis pubis (Chap. 22, Diagnosis). Although some mentum posterior presentations persist, most convert spontaneously to anterior even in late labor (Duff, 1981). If not, the fetal brow (bregma) is pressed against the maternal symphysis pubis. This position precludes flexion of the fetal head necessary to negotiate the birth canal. Thus, a mentum posterior presentation is undeliverable except with a very preterm fetus.

Face presentation is diagnosed by vaginal examination and palpation of facial features. A breech may be mistaken for a face presentation. Namely, the anus may be mistaken for the mouth, and the ischial tuberosities for the malar prominences. Digital differentiation is described in Chapter 28 (Diagnosis). Radiographically, demonstration of the hyperextended head with the facial bones at or below the pelvic inlet is characteristic.

Cruikshank and White (1973) reported an incidence of 1 in 600, or 0.17 percent. As shown in Table 22-1, among more than 70,000 singleton newborns delivered at Parkland Hospital, approximately 1 in 2000 had a face presentation at delivery.

Etiology

Causes of face presentations are numerous and include conditions that favor extension or prevent head flexion. Preterm fetuses, with their smaller head dimensions, can engage before conversion to vertex position (Shaffer, 2006). In exceptional instances, marked enlargement of the neck or coils of cord around the neck may cause extension. Bashiri and associates (2008) reported that fetal malformations and hydramnios were risk factors for face or brow presentations. Anencephalic fetuses naturally present by the face.

Extended neck positions develop more frequently when the pelvis is contracted or the fetus is very large. In a series of 141 face presentations studied by Hellman and coworkers (1950), the incidence of inlet contraction was 40 percent. This high incidence of pelvic contraction should be kept in mind when considering management.

High parity is a predisposing factor for face presentation (Fuchs, 1985). In these cases, a pendulous abdomen permits the back of the fetus to sag forward or laterally, often in the same direction in which the occiput points. This promotes extension of the cervical and thoracic spine.

Mechanism of Labor

Face presentations rarely are observed above the pelvic inlet. Instead, the brow generally presents early and is usually converted to present the face after further extension of the neck during descent. The mechanism of labor in these cases consists of the cardinal movements of descent, internal rotation, and flexion, and the accessory movements of extension and external rotation (Fig. 23-7). Descent is brought about by the same factors as in cephalic presentations. Extension results from the relation of the fetal body to the deflected head, which is converted into a two-armed lever, the longer arm of which extends from the occipital condyles to the occiput. When resistance is encountered, the occiput must be pushed toward the back of the fetus while the chin descends.

The objective of internal rotation of the face is to bring the chin under the symphysis pubis. Only in this way can the neck traverse the posterior surface of the symphysis pubis. If the chin rotates directly posteriorly, the relatively short neck cannot span the anterior surface of the sacrum, which measures about 12 cm in length. Moreover, the fetal brow (bregma) is pressed against the maternal symphysis pubis. This position precludes flexion necessary to negotiate the birth canal. Hence, as discussed earlier, birth of the head from a mentum posterior position is impossible unless the shoulders enter the pelvis at the same time, an event that is impossible except when the fetus is extremely small or macerated. Internal rotation results from the same factors as in vertex presentations.

After anterior rotation and descent, the chin and mouth appear at the vulva, the undersurface of the chin presses against the symphysis, and the head is delivered by flexion. The nose, eyes, brow (bregma), and occiput then appear in succession over the anterior margin of the perineum. After birth of the head, the occiput sags backward toward the anus. Next, the chin rotates externally to the side toward which it was originally directed, and the shoulders are born as in cephalic presentations.

Edema may sometimes significantly distort the face. At the same time, the skull undergoes considerable molding, manifested by an increase in length of the occipitomental diameter of the head.

Management

In the absence of a contracted pelvis and with effective labor, successful vaginal delivery usually will follow. Fetal heart rate monitoring is probably better done with external devices to avoid damage to the face and eyes. Because face presentations among term-size fetuses are more common when there is some degree of pelvic inlet contraction, cesarean delivery frequently is indicated. Attempts to convert a face presentation manually into a vertex presentation, manual or forceps rotation of a persistently posterior chin to a mentum anterior position, and internal podalic version and extraction are dangerous and should not be attempted. Low or outlet forceps delivery of a mentum anterior face presentation can be completed and is described in Chapter 29 (Vacuum Extraction).

Brow Presentation

This rare presentation is diagnosed when that portion of the fetal head between the orbital ridge and the anterior fontanel presents at the pelvic inlet. As shown in Figure 23-8, the fetal head thus occupies a position midway between full flexion (occiput) and extension (face). Except when the fetal head is small or the pelvis is unusually large, engagement of the fetal head and subsequent delivery cannot take place as long as the brow presentation persists.

The causes of persistent brow presentation are the same as those for face presentation. A brow presentation is commonly unstable and often converts to a face or an occiput presentation (Cruikshank, 1973). The presentation may be recognized by abdominal palpation when both the occiput and chin can be palpated easily, but vaginal examination is usually necessary. The frontal sutures, large anterior fontanel, orbital ridges, eyes, and root of the nose are felt on vaginal examination, but neither the mouth nor the chin is palpable.

With a very small fetus and a large pelvis, labor is generally easy. With a larger fetus, it is usually difficult. This is because engagement is impossible until there is marked molding that shortens the occipitomental diameter or more commonly, until the neck either flexes to an occiput presentation or extends to a face presentation. The considerable molding essential for vaginal delivery of a persistent brow characteristically deforms the head. The caput succedaneum is over the forehead, and it may be so extensive that identification of the brow by palpation is impossible. In these instances, the forehead is prominent and squared, and the occipitomental diameter is diminished.

In transient brow presentations, the prognosis depends on the ultimate presentation. If the brow persists, prognosis is poor for vaginal delivery unless the fetus is small or the birth canal is large. Principles of management are the same as those for a face presentation.

Transverse Lie

In this position, the long axis of the fetus is approximately perpendicular to that of the mother. When the long axis forms an acute angle, an oblique lie results. The latter is usually only transitory, because either a longitudinal or transverse lie commonly results when labor supervenes. For this reason, the oblique lie is called an unstable lie in Great Britain.

In a transverse lie, the shoulder is usually positioned over the pelvic inlet. The head occupies one iliac fossa, and the breech the other. This creates a shoulder presentation in which the side of the mother on which the acromion rests determines the designation of the lie as right or left acromial. And because in either position the back may be directed anteriorly or posteriorly, superiorly or inferiorly, it is customary to distinguish varieties as dorsoanterior and dorsoposterior (Fig. 23-9).

A transverse lie is usually recognized easily, often by inspection alone. The abdomen is unusually wide, whereas the uterine fundus extends to only slightly above the umbilicus. No fetal pole is detected in the fundus, and the ballottable head is found in one iliac fossa and the breech in the other. The position of the back is readily identifiable. When the back is anterior, a hard resistance plane extends across the front of the abdomen. When it is posterior, irregular nodulations representing fetal small parts are felt through the abdominal wall.

On vaginal examination, in the early stages of labor, if the side of the thorax can be reached, it may be recognized by the “gridiron” feel of the ribs. With further dilation, the scapula and the clavicle are distinguished on opposite sides of the thorax. The position of the axilla indicates the side of the mother toward which the shoulder is directed.

Transverse lie was found once in 322 singleton deliveries (0.3 percent) at both the Mayo Clinic and the University of Iowa Hospital (Cruikshank, 1973; Johnson, 1964). This is remarkably similar to the incidence at Parkland Hospital of approximately 1 in 335 singleton fetuses.

Etiology

Some of the more common causes of transverse lie include: (1) abdominal wall relaxation from high parity, (2) preterm fetus, (3) placenta previa, (4) abnormal uterine anatomy, (5) hydramnios, and (6) contracted pelvis.

Women with four or more deliveries have a tenfold incidence of transverse lie compared with nulliparas. A relaxed and pendulous abdomen allows the uterus to fall forward, deflecting the long axis of the fetus away from the axis of the birth canal and into an oblique or transverse position. Placenta previa and pelvic contraction act similarly. A transverse or oblique lie occasionally develops in labor from an initial longitudinal position.

Mechanism of Labor

Spontaneous delivery of a fully developed newborn is impossible with a persistent transverse lie. After rupture of the membranes, if labor continues, the fetal shoulder is forced into the pelvis, and the corresponding arm frequently prolapses (Fig. 23-10). After some descent, the shoulder is arrested by the margins of the pelvic inlet. As labor continues, the shoulder is impacted firmly in the upper part of the pelvis. The uterus then contracts vigorously in an unsuccessful attempt to overcome the obstacle. With time, a retraction ring rises increasingly higher and becomes more marked. With this neglected transverse lie, the uterus will eventually rupture. Even without this complication, morbidity is increased because of the frequent association with placenta previa, the increased likelihood of cord prolapse, and the necessity for major operative efforts.

If the fetus is small—usually <800 g—and the pelvis is large, spontaneous delivery is possible despite persistence of the abnormal lie. The fetus is compressed with the head forced against its abdomen. A portion of the thoracic wall below the shoulder thus becomes the most dependent part, appearing at the vulva. The head and thorax then pass through the pelvic cavity at the same time. The fetus, which is doubled upon itself and thus sometimes referred to as conduplicato corpore, is expelled.

Management

Active labor in a woman with a transverse lie is usually an indication for cesarean delivery. Before labor or early in labor, with the membranes intact, attempts at external version are worthwhile in the absence of other complications. If the fetal head can be maneuvered by abdominal manipulation into the pelvis, it should be held there during the next several contractions in an attempt to fix the head in the pelvis.

With cesarean delivery, because neither the feet nor the head of the fetus occupies the lower uterine segment, a low transverse incision into the uterus may lead to difficult fetal extraction. This is especially true of dorsoanterior presentations. Therefore, a vertical hysterotomy incision is often indicated.

Compound Presentation

With this, an extremity prolapses alongside the presenting part, and both present simultaneously in the pelvis (Fig. 23-11). Goplerud and Eastman (1953) identified a hand or arm prolapsed alongside the head once in every 700 deliveries. Much less common was prolapse of one or both lower extremities alongside a cephalic presentation or a hand alongside a breech. At Parkland Hospital, compound presentations were identified in only 68 of more than 70,000 singleton fetuses—an incidence of approximately 1 in 1000. Causes of compound presentations are conditions that prevent complete occlusion of the pelvic inlet by the fetal head, including preterm labor.

In most cases, the prolapsed part should be left alone, because most often it will not interfere with labor. If the arm is prolapsed alongside the head, the condition should be observed closely to ascertain whether the arm retracts out of the way with descent of the presenting part. If it fails to retract and if it appears to prevent descent of the head, the prolapsed arm should be pushed gently upward and the head simultaneously downward by fundal pressure.

In general, rates of perinatal mortality and morbidity are increased as a result of concomitant preterm delivery, prolapsed cord, and traumatic obstetrical procedures. Serious injury to the forearm is rare (Kwok, 2015; Tebes, 1999).

Maternal Complications

Dystocia, especially if labor is prolonged, is associated with a higher incidence of several common obstetrical and neonatal complications. Infection, either intrapartum chorioamnionitis or postpartum pelvic infection, is more common with desultory and prolonged labors. Postpartum hemorrhage rates from atony are increased with prolonged and augmented labors. Uterine tears with hysterotomy also occur at greater incidence if the fetal head is impacted in the pelvis.

Uterine rupture is another risk. Abnormal thinning of the lower uterine segment creates a serious danger during prolonged labor, particularly in women of high parity and in those with a prior cesarean delivery. When disproportion is so pronounced that there is no engagement or descent, the lower uterine segment becomes increasingly stretched, and rupture may follow. In such cases, the normal contraction ring is usually exaggerated, like that shown in Figure 23-10.

Such pathological retraction rings are localized constrictions of the uterus that develop in association with prolonged obstructed labors. Seldom encountered today, the pathological retraction ring of Bandl is associated with marked stretching and thinning of the lower uterine segment. In contemporary practice, after birth of a first twin, a pathological ring may still develop occasionally as an hourglass constriction of the uterus. The band may be seen clearly as a uterine indentation and signifies impending rupture of the lower uterine segment. The ring can sometimes be relaxed and delivery effected with appropriate general anesthesia, but occasionally prompt cesarean delivery offers a better prognosis for the second twin (Chap. 45, Vaginal Birth after Cesarean Delivery).

Fistula formation may result from dystocia, as the presenting part is firmly wedged into the pelvic inlet. Tissues of the birth canal lying between the leading part and the pelvic wall may be subjected to excessive pressure. Because of impaired circulation, necrosis can result and become evident several days after delivery as vesicovaginal, vesicocervical, or rectovaginal fistulas. Most often, pressure necrosis follows a very prolonged second stage. Such fistulas are rarely seen today except in undeveloped countries.

Pelvic floor injury during pregnancy and childbirth has gained recent attention. The pelvic floor is exposed to direct compression from the fetal head and to downward pressure from maternal expulsive efforts. These forces stretch and distend the pelvic floor, resulting in functional and anatomical alterations in the muscles, nerves, and connective tissues. Accumulating evidence suggests that such effects on the pelvic floor during childbirth can affect urinary or anal continence and pelvic support. These relationships are discussed in Chapter 30 (Cesarean Delivery Risks).

Lower extremity nerve injury in the mother can follow prolonged second-stage labor. Wong and colleagues (2003) reviewed neurological injury involving the lower extremities in association with labor and delivery. The most common mechanism is external compression of the common fibular (formerly common peroneal) nerve. This is usually caused by inappropriate leg positioning in stirrups, especially during prolonged second-stage labor. These and other injuries are discussed in Chapter 36 (Pain, Mood, and Cognition). Fortunately, symptoms resolve within 6 months of delivery in most women.

Perinatal Complications

Similar to the mother, the incidence of peripartum fetal sepsis rises with longer labors. Caput succedaneum and molding develop commonly and may be impressive (Fig. 22-16) (Buchmann, 2008). Mechanical trauma such as nerve injury, fractures, and cephalohematoma are also more frequent and are discussed further in Chapter 33 (Injuries of the Newborn).