Though attempts at a trial of labor after a cesarean birth have become accepted practice, the rate of successful vaginal birth after cesarean delivery (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 delivery, they are less likely to allow new patients to undergo a trial of labor. In addition, 1999 guidelines from ACOG clearly state that patients undergoing a trial of labor after cesarean delivery require the presence of an obstetrician, an anesthesiologist, and/or a staff capable of performing an emergency cesarean delivery throughout the patient’s active phase of labor. While academic centers and larger community hospitals readily comply with this requirement, 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 national birth statistics. The cesarean delivery rate peaked and dropped from 26% to 21% overall in 1996. However, from 1996-1999, the cesarean delivery rate increased, while the rate of VBAC declined from 28% to 23%. Assuming an overall 70% success rate, this correlates with a decline of 40% to 33% in the number of patients with a prior cesarean birth 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 birth) is losing the support it had in the 1980s.
When considering which patients should be offered a trial of labor after cesarean delivery, 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 a trial of labor. Many practices and institutions have adopted a separate consent form for patients wishing to undergo an attempt at VBAC. While 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.
Two specific outcomes of interest regarding a trial of labor after cesarean delivery have been well investigated: successful VBAC and uterine rupture. Other outcomes are certainly of interest, including neonatal outcome, hysterectomy, and maternal mortality; however, few studies have focused on these outcomes, and poor outcomes occur too rarely to be well represented in established databases.
In earlier studies, most outcomes were reported after univariate analysis. Risk factors were examined without controlling for other possible risk factors, and results were reported as a relative risk or odds ratio. These ratios represent the risk of the group of individuals who have the risk factor divided by individuals without the risk factor. Most of the studies over the last decade have used multivariate statistics to examine risk factors. This means that other risk factors and confounding factors, such as birthweight, maternal age, obstetric history, and labor management, were controlled in the analysis.
PREDICTORS OF A SUCCESSFUL TRIAL OF
Because no controlled trials have been conducted, most of the risk factors have been determined from retrospective cohort and case-control studies. These studies have been increasingly analyzed 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.
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 is usually 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 is also 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 delivery.
Maternal age has also been examined in nulliparous women. Studies have demonstrated that women older than 35 years have higher rates of cesarean delivery. Maternal age has also been examined in several studies in the VBAC literature. Adjusting for confounding factors, women older than 40 years who have had a prior cesarean delivery have an almost 3-fold higher risk for a failed trial of labor 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.
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 higher 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 is enormously important in terms of risk factors for a successful trial of labor. Predictors for increased success include a nonrecurrent indication for prior cesarean delivery (eg, breech birth) and prior vaginal delivery. A history of cephalopelvic disproportion (CPD), failure to progress, no prior vaginal deliveries, or a prior cesarean delivery performed in the second stage of labor are negative predictors of success in a subsequent trial of labor.
Indications for prior cesarean delivery
Several studies have examined indications for prior cesarean delivery 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%). Nonrecurrent 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 delivery are also associated with a higher proportion of patients not attempting a trial of labor after cesarean birth. In a meta-analysis of the existing literature prior to 1990, Rosen demonstrated that women whose prior cesarean delivery 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 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 a 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 delivery
Only one study carefully examines cervical dilation at prior cesarean delivery. 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 has also 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 delivery have a 2- to 3-fold increased risk of cesarean delivery compared to those who present with spontaneous labor.
Cervical examination on admission
Not surprisingly, patients who present to labor and delivery with advanced cervical examination findings have a greater success rate of vaginal birth. Several components of the cervical examination have been investigated, including cervical dilation and cervical effacement. Not surprisingly, the more advanced the cervical examination finding is upon initial presentation, the higher the rate of successful VBAC. Flamm et al demonstrated that patients presenting with dilation greater than or equal to 4 cm had an 86% rate of VBAC.
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 are often delivered at 36-37 weeks' gestation. Though available data are limited, the risk of uterine rupture in this group of patients is approximately 6-12%.
Low vertical hysterotomy (Krönig)
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 is 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 abdominally are delivered through a transverse incision in the lower uterine segment (Kerr hysterotomy). In several large retrospective cohort studies, the reported rate of uterine rupture is 0.5-1.0%.
Rates of Uterine Rupture
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 delivery for a breech presentation at 28 weeks’ gestation has a much higher risk of a vertical uterine incision than the patient at term with arrest of dilation. Because most cesarean deliveries are via low transverse hysterotomies, the risk of uterine rupture for patients with an unknown uterine scar is usually similar to that of patients 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 compared to low transverse hysterotomy.
Number of prior cesarean deliveries
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 cesarean deliveries have 5 times the risk of uterine rupture compared to patients who have had only 1 prior cesarean delivery (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 a 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 the rate of uterine rupture in patients with a prior VBAC to those with a vaginal delivery before their prior cesarean delivery.
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 are usually 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 over only 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 were 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
One randomized trial examined the use of oxytocin augmentation of early labor compared to expectant management. In this study, 5 uterine ruptures occurred 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 statistically associated with uterine rupture, although the odds ratio was 2.3 (95% confidence interval [CI], 0.8-7.1) in the latter study. 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 have been conducted, the 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% CI, 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 3-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, particularly misoprostol.
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 delivery. 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 delivery is difficult to assess because it is not commonly investigated. Thus, determining whether these results are different from the baseline rate in singleton gestations is difficult.
In 1999, one small series by Ravasia et al examined patients with müllerian anomalies undergoing a trial of labor after cesarean delivery. In this series, 2 uterine ruptures occurred among 25 patients (8%). However, both patients with uterine rupture had undergone induction with prostaglandins; therefore, making any assessment about the relative risk of uterine rupture in this group of patients is difficult. The sample size in this study was certainly too small to consider particular types of uterine anomalies.
MANAGEMENT OF PATIENTS WITH PRIOR
Once the patient presents to labor and delivery and is in labor, several practices can help minimize the maternal and neonatal risk. 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. Harbingers of uterine rupture are as follows:
Treat any of these findings as uterine rupture until another source for the finding has been identified. Rupture requires immediate delivery.
The decision to undergo a trial of labor after cesarean delivery is an individual one that should be based on careful, thorough counseling. Maternal characteristics and obstetric history can provide a patient a rough estimate of her chance of a successful trial of labor. This same obstetric history can be used to estimate a patient's risk of uterine rupture. If possible, avoid induction of labor because induction of labor decreases the probability of success and increases the chance of uterine rupture in a trial of labor after cesarean delivery. Counsel patients who elect to undergo a trial of labor after cesarean delivery to be evaluated early in labor and to manage the pregnancy in a hospital setting in which uterine rupture can be both recognized and managed expediently.