Vaginal Birth After Cesarean

INTRODUCTION

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In 1916, Cragin said, "Once a cesarean, always a cesarean." These words spoken to the New York Association of Obstetricians & Gynecologists reflected obstetricians' management of patients with a prior cesarean birth in the US for the next 60 years. By 1988, the overall cesarean rate was 25%, rising from less than 5% in the early 1970s. Only 3% of live-born infants were delivered vaginally after the mother had undergone a prior cesarean section. To curb the increasing rate of cesarean birth, both the National Institutes of Health (NIH) and the American College of Obstetricians and Gynecologists (ACOG) issued statements encouraging obstetricians to support a trial of labor in patients who had undergone a prior cesarean delivery.

Though attempts at a trial of labor after cesarean have become accepted practice, the rate of successful vaginal births after cesarean (VBAC) has decreased during the past 10 years. Several factors have contributed to this decline. As practitioners experience complications related to managing patients undergoing trials of labor after cesarean, they are less likely to allow new patients to undergo a trial of labor. In addition, guidelines from ACOG in 1999 clearly stated that patients undergoing a trial of labor after cesarean require the presence of an obstetrician, an anesthesiologist, and/or staff capable of performing an emergency cesarean delivery throughout the patient’s active phase of labor. While academic centers and larger community hospitals comply readily with these requirements, many smaller hospitals do not offer in-house anesthesia. Furthermore, to meet demands of managed care, many obstetricians now cover more than one hospital, making it difficult to meet the ACOG guidelines.

The impact of these changes can be observed in the national birth statistics. The cesarean rate peaked and dropped from 26-21% overall in 1996. However, from 1996-1999, the rate of cesarean delivery increased, while the rate of VBAC has declined from 28-23%. Assuming an overall 70% success rate, this correlates with a decline from 40-33% in the number of patients with a prior cesarean choosing to undergo a trial of labor. What once was hailed as a key component of lowering the overall cesarean birth rate (ie, trial of labor in patients with previous cesarean) is losing the support it had in the 1980s.

When considering which patients should be offered a trial of labor after cesarean, ensure that compliance with ACOG recommendations can be met. Once compliance is established, routinely counsel patients early in the pregnancy regarding the risks and benefits of trial of labor. Many practices and institutions have adopted a separate consent for patients wishing to undergo an attempt at VBAC. While this consent helps to formalize counseling, documentation of the overall risks quoted to the patient, specifically mentioning the individual's risk factors, is all that is necessary.

When examining the literature regarding trial of labor after cesarean, 2 specific outcomes of interest have been well investigated—successful VBAC and uterine rupture. Certainly, other outcomes are of interest, including neonatal outcome, hysterectomy, and maternal mortality. However, few studies have focused on these outcomes, and the poor outcomes occur too rarely to be well studied in the established databases.

PREDICTORS OF A SUCCESSFUL TRIAL OF LABOR

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Existing literature extensively describes the predictors of a successful trial of labor after cesarean delivery. While no randomized trials exist, relatively large databases have been analyzed using both univariate and multivariate techniques to determine risk factors for patients undergoing a trial of labor after cesarean. In addition, several scoring systems have been devised to help predict which patients are likely to be successful when attempting a VBAC.

Because no controlled trials exist, most of the risk factors have been determined from retrospective cohort and case-control studies. These studies have been analyzed increasingly with multivariate techniques, allowing for control of confounding factors. However, control for physician practice is difficult, and physician practice can greatly impact the strength of the association between these risk factors and a successful VBAC. Even when controlling for demographic data, obstetric history, birthweight, gestational age, induction of labor, and other variables, the bias inherent in nonrandomized and nonblinded trials cannot be eliminated.

Maternal characteristics

Because large studies examining the association of maternal height or weight with successful VBAC have not been conducted, retrospective cohort studies in nulliparous women substitute for these studies. Maternal height usually is dichotomized to greater than or less than 1.5 m (4 ft, 11 in) or 1.6 m (5 ft, 3 in). In these studies, taller patients have greater rates of vaginal delivery, with odds ratios ranging from 1.3-3.2. Maternal weight also is dichotomized but in many different ways. Both 150% of ideal body weight and absolute weight from 200-300 lb have been used as the threshold. In this heterogeneous group of studies, the odds ratios range from 1.6-2.8 for heavier patients to have higher rates of cesarean section.

Maternal age also has been examined in nulliparous women. Studies have demonstrated that women older than 35 years have higher rates of cesarean delivery. Maternal age also has been examined in several studies in the VBAC literature. Adjusting for confounding factors, women older than 40 years who have had a prior cesarean section have an almost 3-fold risk for a failed trial of labor when compared to women younger than 40 years. In one scoring system, Flamm et al gave women younger than 40 years an extra point as a predictor for successful VBAC.

Birthweight

Because of a paucity of data on birthweight and VBAC, available information is extrapolated from studies in nulliparous women. Birthweight greater than 4000 g is associated with an almost 4-fold risk of cesarean birth (odds ratio 2.3-3.7). Several studies have demonstrated a difference in VBAC rates between patients with a birthweight greater than 4000 g and those with a lower birthweight. The odds ratio for this risk factor ranges from 1.2-1.9.

Obstetric history

Obstetric history is enormously important in terms of risk factors for a successful trial of labor. Predictors for increased success include a nonrecurring indication for prior cesarean (eg, breech birth) and prior vaginal delivery. A history of cephalopelvic disproportion (CPD), failure to progress, no prior vaginal deliveries, or a prior cesarean section performed in second stage are negative predictors of success in a subsequent trial of labor.

Indication for prior cesarean

Several studies have examined indications for prior cesarean as a predictor of outcome in a subsequent trial of labor. In all studies, CPD had the lowest VBAC success rate (60-65%). Fetal distress (eg, nonreassuring fetal testing) had the second lowest rates of VBAC (69-73%). Nonrecurring indications, such as breech birth, herpes, and placenta previa, were associated with the highest rates of success (77-89%). Failure to progress, CPD, or dystocia as indications for prior cesarean also are associated with a higher proportion of patients not attempting a trial of labor after cesarean. In a meta-analysis of the existing literature prior to 1990, Rosen demonstrated that women whose prior cesarean was performed for CPD were twice as likely to have an unsuccessful trial of labor.

Prior vaginal delivery

Patients with a prior vaginal delivery have higher rates of successful VBAC when compared to patients without a prior vaginal birth. Furthermore, women with a successful VBAC have a higher success rate in a subsequent trial of labor compared to women whose vaginal delivery was prior to cesarean delivery. In an unadjusted comparison, patients with 1 prior vaginal delivery had an 89% VBAC success rate compared to 70% success rate in patients without a prior vaginal delivery. In comparisons controlling for confounding factors, odds ratios of 0.3-0.5 for rate of cesarean delivery are found when comparing patients with a prior vaginal delivery to those without prior vaginal delivery. In patients with a prior VBAC, Caughey et al found that the success rate is 93% compared to 85% in patients with a vaginal delivery prior to their cesarean birth but no prior VBAC.

Cervical dilation at prior cesarean

Only one study carefully examines cervical dilation at prior cesarean. In this study, the degree of cervical dilation in the prior delivery is directly associated with the likelihood of success in the subsequent trial of labor. For example, 67% of patients who were dilated 5 cm or less at the time of their delivery had a successful VBAC compared to 73% of patients who were dilated 6-9 cm. The success rate is much lower for patients whose labor arrested in the second stage; in one study, only 13% of patients who were fully dilated at the time of their prior delivery had a successful VBAC.

Induction of labor

Patients who undergo induction of labor are at a higher risk of cesarean delivery compared to women who experience spontaneous labor. This finding also has been observed in women with a prior cesarean delivery. Several studies have demonstrated that women who are induced in a trial of labor after cesarean have a 2- to 3-fold increased risk of cesarean compared to those who present with spontaneous labor.

Cervical exam on admission

RISK FACTORS FOR UTERINE RUPTURE

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One of the most significant risks women face when considering a trial of labor is that of uterine rupture. This potentially fatal event may have significant maternal and neonatal sequelae. A threshold of acceptable risk has been established between risk of women with 1 prior cesarean (0.5-1%) and women with a history of a prior classic cesarean (6-12%). These latter patients, along with women who have had metroplasty surgery for uterine anomalies or myomectomies that have entered the uterine cavity, are discouraged from attempting a VBAC. Other patients who are at increased risk for uterine rupture include those who have had 2 or more hysterotomies, those who use prostaglandin agents, and patients undergoing induction of labor.

Classic hysterotomy

Unquestionably, practitioners do not feel safe allowing a patient who has had a prior classic hysterotomy (ie, a vertical incision that has extended above the insertion of the round ligaments) to undergo a trial of labor. Patients with a prior classic hysterotomy have a higher rate of uterine rupture in subsequent pregnancies. Because these patients can sustain a uterine rupture prior to labor, they often are delivered at 36-37 weeks' gestation. Though data available is limited, the risk of uterine rupture in this group of patients is approximately 6-12%.

Low vertical hysterotomy

Both Adair and Shipp demonstrated in retrospective cohort studies that the risk of uterine rupture is no greater for patients who have had a vertical incision in the lower uterine segment than those who have had a transverse incision. The rate of uterine rupture from these studies ranged 0.8-1.3%. When comparing patients with prior Krönig hysterotomies to patients with low transverse incisions, no statistical difference exists in either univariate or multivariate analyses controlling for the confounding factors of obstetrical history, induction of labor, birthweight, and length of labor.

Low transverse hysterotomy (Kerr)

Most babies delivered by cesarean section are delivered through a transverse incision in the lower uterine segment (Kerr hysterotomy). In several large retrospective cohort studies, the rate of uterine rupture ranges 0.5-1.0%.

Rates of Uterine Rupture

*Includes unknown

Unknown hysterotomy

When an obstetrician cannot obtain an operative report of a patient's prior cesarean delivery, obstetric history may be helpful in determining the type of uterine incision. For example, a patient who underwent a cesarean for a breech presentation at 28 weeks has a much higher risk of a vertical uterine incision than does the patient at term with arrest of dilation. Because most cesarean deliveries are via low transverse hysterotomies, in most cases, the risk of uterine rupture for patients with an unknown uterine scar will be similar to those who have had a prior transverse incision. Several studies examining this issue have demonstrated that the rate of rupture for patients with an unknown uterine incision is approximately 0.6%. Leung conducted a case-control study of patients with and without uterine rupture and did not find unknown hysterotomy to be a risk factor when compared to low transverse hysterotomy.

Number of prior cesareans

Patients with more than 1 prior cesarean delivery are at increased risk of uterine rupture. The unadjusted rate of uterine rupture for patients with 2 prior uterine incisions ranges from 1.8-3.7%. Caughey et al demonstrated that when potential confounding variables (eg, prior vaginal delivery) are controlled for, patients who have had 2 prior cesareans have 5 times the risk of uterine rupture compared to patients who have had only 1 prior cesarean (odds ratio 4.8). This finding contradicted several earlier studies that did not control for confounding factors, most importantly prior vaginal delivery.

Prior vaginal delivery

Only one study carefully examines prior vaginal delivery as a variable. Zelop et al demonstrated that patients with a prior vaginal delivery had 0.2% rate of rupture compared to 1.1% for patients with no prior vaginal delivery. An adjusted odds ratio controlling for confounding factors was 6.2. No studies have compared rate of uterine rupture in patients with a prior VBAC to those with a vaginal delivery before their prior cesarean.

Labor

While labor appears to be a risk factor for uterine rupture, many patients experience a uterine rupture prior to the onset of labor. In a large study using birth certificate data, Gregory et al found that the rate of uterine rupture prior to the onset of labor was 0.5%. Patients at greatest risk are those with prior classic hysterotomies. As a result of this potential risk, these patients usually are scheduled for delivery at 36-37 weeks' gestation. When counseling this group of patients, mention that scheduling an early repeat cesarean delivery does not eliminate the risk of uterine rupture entirely.

Induction of labor

Induction of labor as a risk factor for uterine rupture has been examined only over the past 5 years. One large study, which did not control for confounding factors, demonstrated a uterine rupture rate of 2.3% in patients who had experienced induced labor. In several studies controlling for confounding factors, adjusted odds ratios ranged from 2-4 for patients who were induced compared to those who presented in spontaneous labor. Despite these analyses, discerning whether induction of labor itself leads to uterine rupture or whether another risk factor (as yet unmeasured) is associated with both induction of labor and uterine rupture is difficult. When patients have an indication for induction of labor, the best course of management is to discuss the increase over their baseline risk of uterine rupture.

Use of oxytocin

There is one randomized trial that examined the use of oxytocin augmentation of early labor as compared to expectant management. In this study, there were 5 uterine ruptures in the augmentation group and none in the expectant management group. Excessive use of oxytocin has been described as leading to an increased risk of uterine rupture. Leung et al demonstrated an odds ratio of 2.7 for uterine rupture in patients who used oxytocin compared to those without oxytocin augmentation. However, this study included inductions and augmentation in both the latent and active phases. These confounding factors may have contributed to much of the difference because induction of labor alone has an increased risk. In contrast, 2 subsequent studies from Rageth and Zelop demonstrated that oxytocin augmentation was not associated statistically with uterine rupture. Though in the latter study the odds ratio was 2.3 (95% CI 0.8 - 7.1). Because labor and delivery units use oxytocin widely, this oxytocin-uterine rupture relationship deserves careful consideration; the relationship directly impacts management of patients. At this point it is probably advisable to use oxytocin for induction and augmentation only when absolutely necessary.

Use of prostaglandins

Evidence regarding use of prostaglandins for induction of labor remains scant. However, in the few trials that exist, a trend is toward an increase in the rate of uterine rupture. In 1999, the use of prostaglandin E2 was demonstrated to have an adjusted odds ratio of 3.2 (95% confidence interval 0.9-10.9). This study controlled for induction of labor, which appears to be an independent risk factor. In 2001, Lydon-Rochelle et al demonstrated a three-fold increase in the risk for uterine rupture when comparing patients induced with prostaglandins to those induced with oxytocin. As in studies examining the risk of uterine rupture in patients undergoing induction of labor, patients who were treated with prostaglandins were likely to have other confounding variables that were not controlled. In particular, the prostaglandin Misoprostol has been examined in small studies. In 1998, Wing et al reported a case series of 17 patients who were induced with misoprostol, in which 2 uterine ruptures occurred. These findings have led to the decreased use of prostaglandins for induction, and in particular Misoprostol.

Twin gestations

Two small studies by Miller and Strong examine the rate of uterine rupture in patients with twin gestation undergoing a trial of labor after cesarean. Neither study demonstrated a frank rupture, although combining their data revealed 5 asymptomatic dehiscences in 117 patients. The rate of asymptomatic uterine dehiscence in patients undergoing trial of labor after cesarean is difficult to assess because it is not investigated commonly. Thus, determining whether these results are different from the baseline rate in singleton gestations is difficult.

Müllerian anomalies

MANAGEMENT OF PATIENTS WITH PRIOR CESAREAN DELIVERIES

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Patients with prior cesarean deliveries need special management both antenatally as well as in labor and delivery. Early in their prenatal care, catalog patients' preexisting risk factors for both successful VBAC and uterine rupture. If uncertainty surfaces regarding the prior hysterotomy, obtain the operative note from their cesarean delivery. When all obstetric history is obtained, counsel patients regarding the risks and benefits of undergoing a trial of labor after cesarean and plan the particular mode of delivery with patients. Again, some providers and hospitals have particular consents that must be signed at this point regarding trial of labor after cesarean.

Once the patient presents to labor and delivery and is in labor, several points can help minimize the maternal and neonatal risk. As per ACOG guidelines, have an obstetrician, anesthesiologist, and operating room team readily available to provide an emergent delivery. Clinically, observe the patient closely for signs of uterine rupture. A few harbingers of uterine rupture include the following: