Abstract
Vaginal birth after cesarean (VBAC) has been a research topic of widespread interest in the last decade, with research focus on characterizing risks and benefits and identifying prognostic factors. The latest data regarding VBAC assesses outcomes in an important subpopulation of patients, namely VBAC candidates with multiple prior successful VBAC attempts [1]. The rate of cesarean delivery in the USA – and in most other developed countries – has fluctuated dramatically over the past 30 years. Between 1970 and 1988, the cesarean delivery rate in the USA rose from 5 to nearly 25% until the advent and wide-spread utilization of VBAC, which was accompanied by a decrease in the cesarean rate to 20.7% in 1996 [2]. However, as VBAC experience increased and cases of VBAC-related adverse events were reported, many physicians became concerned about the risk of uterine rupture in patients attempting VBAC. In response to these concerns the VBAC rate plummeted and the cesarean rate began increasing again, peaking at 30.2% in 2005 [3]. The increasing rate of cesarean delivery is worrisome from both a public health and individual patient perspective. Patients who undergo cesarean delivery not only have surgical risks in the index pregnancy, but also incur increasing rates of maternal and neonatal morbidity with an increasing number of cesareans. Risks of placenta accreta, maternal bowel injury, ureteral injury, cystotomy, hysterectomy and intensive care admission persistently rise with increasing number of cesarean deliveries [4]. Thus, a policy that universally encourages repeat cesarean in lieu of VBAC could induce a multiplicative rise in delivery complications and a substantial public health burden by dramatically increasing the number of highest-risk patients with multiple cesareans.
Given the maternal morbidity associated with increasing number of cesareans, attempts have been rekindled to offer women with a history of a low transverse cesarean a trial of labor to attempt a vaginal birth. The American College of obstetricians and Gynecologists (ACOG) states that women with a history of one previous low transverse cesarean delivery, a clinically adequate pelvis, and no prior classical uterine scar or rupture are good candidates for a VBAC trial provided that they are at an institution with adequate resources including physicians and anesthesiologists [2]. Two recent large observational studies have shown that the VBAC success rate is 73.4–75.5% [4,8] with a 0.7–0.9% risk of symptomatic uterine rupture [5,7]. Successful VBAC has been associated with several benefits, including shorter maternal hospitalizations, less blood loss, fewer infections and fewer thromboembolic events compared with repeat cesarean [2]. While it has been shown that VBAC is safer when successful, clinicians and patients are still concerned about catastrophic obstetrical outcomes that are most commonly related to or attributed to VBAC failure. Unfortunately, attempts to predict VBAC failure have only been moderately successful and predicting uterine rupture in patients attempting VBAC appears to be untenable largely as a result of the rarity of the morbid event [8–10]. Although VBAC failure or uterine rupture has not been consistently and reliably predicted, several important risk or protective factors have been identified, which can aid the clinician in counseling. The need for labor induction, increased maternal age, a recurrent cesarean indication (e.g., cephalopelvic disproportion) and non-Caucasian race all increase the risk of an unsuccessful VBAC; while a prior vaginal delivery and a prior VBAC are strongly associated with a successful VBAC attempt [8,9].
What has not been elucidated is the impact of the number of previous VBACs on subsequent pregnancy outcomes, including uterine rupture. A long-standing, but unproven concern has been that multiple VBACs (i.e., beyond one) may actually be associated with increased maternal and neonatal morbidity. This fear was based on the biologically plausible hypothesis that an increasing number of labor and vaginal deliveries after the initial cesarean may produce additive strain on the uterine scar. Thus, characterizing the association between multiple VBACs and delivery mode-related maternal and neonatal outcomes is an important research topic.
Results
In the February 2008 issue of Obstetrics & Gynecology, Mercer and colleagues reported a secondary analysis of a prospective observational cohort study that investigated the association between the number of prior successful VBAC attempts and VBAC-related maternal and neonatal outcomes [1]. The study was conducted at 19 academic medical centers of the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network (NICHD-MFMU Network) between 1999 and 2002 [1]. Women with singleton pregnancies who had previously undergone at least one cesarean delivery by a low transverse uterine incision and who attempted a VBAC productive of a neonate at least 20 weeks gestation or 500 g were included. Data on demographic characteristics, medical and obstetrical history, labor characteristics, delivery outcomes and peripartum complications were reviewed and compiled.
During the study period, 13,532 women with a history of one or more low transverse cesarean delivery attempted a VBAC with an overall success rate of 71.8%. Patients were grouped according to the number of prior VBACs – zero, one and two or more and outcomes were compared across the three study groups. Maternal age, and frequencies of African–American race, greater than one prior cesarean, lower education level and government-assisted health insurance increased with increasing number of VBACs, but other maternal characteristics were similar across the study groups.
The rate of VBAC success rose with increasing number of prior VBACs. Women with no prior VBAC had a 63.3% vaginal delivery rate (95% CI: 62.3–64.3%); women with one prior VBAC had a rate of 87.6% (95% CI: 86.4–88.8%) and those with two or more VBACs had a rate of 90.9% (95% CI: 89.5–92.3%). The rate of uterine rupture, defined as a through and through disruption or tear of the uterine muscle and visceral peritoneum or a uterine muscle separation with extension to adjacent structures, decreased after the first successful VBAC and remained similarly low as the VBAC number increased from two to four or more. Women with no previous VBACs had a uterine rupture risk of 0.87% (95% CI: 0.68–1.07%) while women with one previous VBAC had a rupture risk of 0.45% (95% CI: 0.24–0.76%). Women with two or more VBAC had a uterine rupture risk of 0.43% (95% CI: 0.20–0.85%).
Other maternal morbidities including uterine dehiscence (disruption of the uterine muscle with intact serosa), hysterectomy, thromboembolism, surgical complications and endometritis all appeared to decrease with increasing number of VBACs, though the difference in hysterectomy and thromboembolism risks did not reach statistical significance. As the number of successful VBACs increased, there was no increase in infant death or neonatal morbidity, including neonatal acidemia, neonatal intensive care unit admission and hypoxic ischemic encephalopathy.
Significance
Although the association between prior VBAC and delivery outcomes has been studied previously, the work by Mercer and colleagues represents the largest study with the most detailed characterization of this exposure-outcome relationship [1,11]. Contrary to prior unsubstantiated fears, the results of this study indicate that the rate of uterine rupture decreases by approximately 50% after the initial successful VBAC and does not increase thereafter with increasing number of VBACs. Similarly, the rate of other peripartum major morbid events decreases after the initial successful VBAC. The VBAC success rate improved incrementally with the increasing number of successful VBACs from a low rate of 63% in patients with no prior VBAC to nearly 92% for those with four or more prior VBACs. Importantly, neonatal morbidity rates did not appear to increase with increasing VBAC number.
This study shows that VBAC is a practical and relatively safe and effective option for patients with a prior low transverse cesarean delivery regardless of the number of prior successful VBAC attempts. The results of this study also corroborate prior studies that demonstrated a decrease in maternal and neonatal morbidities with VBAC than compared with repeat cesarean among patients that had a prior vaginal delivery[6]. This new information is vital to help clinicians accurately and effectively counsel patients interested in a vaginal birth after having a previous low transverse cesarean. Until now, patients with multiple prior successful VBAC attempts may have been dissuaded by clinicians from attempting VBAC based on erroneous concerns of increased morbidity.
The study by Mercer and colleagues has several attributes, including a large sample size that allows investigation of rare but clinically important outcomes; a multicenter design that improves generalizability of results; strictly defined outcomes, which should minimize misclassification bias; prospective, structured data collection, which should decrease potential for ascertainment bias; and a robust database that allows for assessing and controling confounding [1]. However, the study is not without limitations. Even though the sample size was relatively large, the power to detect differences in very rare outcomes, such as infant or maternal death, hypoxic ischemic encephalopathy and maternal thromboembolism, is limited. A randomized trial of VBAC would be methodologically ideal, but not be societally acceptable; thus the investigators selected the most methodologically rigorous study design possible – a prospective cohort study. This study design has the potential for selection bias, which can produce erroneous results. Importantly, physicians generally select patients for VBAC or cesarean based on known or perceived risk factors, many of which are employed currently for clinical decision making. Thus, while selection bias in this scenario may have the potential to underestimate VBAC-associated risks in general, it is unlikely that the bias would be differential across study groups or of a sufficient magnitude to invalidate the results. Another limitation is that not all VBAC failure risk factors identified in prior studies are included in their analysis. The most notable is the indication for the prior cesarean. Though this study did not stratify patients based on cesarean indication in the previous pregnancy, it clearly demonstrates a high rate of vaginal delivery with a history of successful VBACs. Thus, a history of a successful VBAC may be a stronger prognostic indicator than the indication for the original cesarean delivery. Lastly, the authors aptly point out that the study results, derived from tertiary academic centers, are likely not generalizable to all healthcare facilities since resources vary by hospital type. However, they also correctly identify that their results are likely applicable to most hospitals with the assets needed and recommended by ACOG to perform VBAC attempts [1,2].
Uterine rupture and other delivery-related catastrophic morbidities are rare events and attempts to accurately predict their occurrence have proven ineffective. As successful VBAC attempts result in less morbidity than repeat cesarean delivery, attempts should be made to identify candidates that are most likely to have a VBAC success, consequently reaping the benefits of vaginal delivery and avoiding the risks of cesarean after a failed VBAC. Women with a history of one or more successful VBAC attempt are one such group of favorable candidates.
In summary, this study adds to a growing body of evidence indicating that the VBAC success rate increases with prior vaginal delivery or successful VBAC attempt and, more importantly, remains a relatively safe option for mothers and their babies, regardless of the number of prior VBACs. Clinicians should be aware that the absolute risks of uterine rupture and other major morbid events remain low and that patients with a history of multiple prior VBACs should not be dissuaded from attempting a vaginal delivery.
Future direction
Observational studies such as this one performed by Mercer and the Maternal–Fetal Medicine Units Network are important as they help to characterize the risks and benefits of competing strategies in the healthcare of patients with a prior Cesarean. While recent studies have helped to identify subpopulations of patients that are most likely to successfully complete a VBAC trial, or conversely, who are at highest risk for uterine rupture, there is still potential to improve maternal and neonatal outcome by enhancing our ability to predict which VBAC candidates will most likely achieve a vaginal delivery without complications. Since major morbid events such as uterine rupture are rare, it is unlikely that further research will improve our ability to predict and directly avoid these complications. However, it is known that VBAC-related major morbid events are mostly attributed to patients that must undergo a difficult labor or require a cesarean after a failed VBAC attempt. For this reason, it is paramount to continue research to refine our ability to predict and avoid failed VBAC attempts to garner the most benefits from a VBAC policy while minimizing adverse outcomes.
Executive summary
Vaginal birth after cesarean (VBAC) success rates increase with an increasing number of prior VBACs.
Maternal peripartum morbidities, including uterine rupture, are decreased in patients with one or more prior successful VBAC.
Neonatal morbidity does not appear to increase with increasing number of prior successful VBACs.
Uterine rupture is a rare event and attempts to predict its occurrence have been unsuccessful.
Future studies should attempt to refine our ability to predict and avoid failed VBAC attempts to minimize VBAC complications.
Footnotes
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.