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After declining to a rate of 20.7% in 1996,1 the frequency of cesarean delivery in the United States has progressively increased over the past decade to 30.2% in 2005, with the majority of these (71%) being the first “primary” cesarean.2 Because of this trend, an increasing number of women are faced with the important decision of whether or not to attempt a vaginal birth after cesarean delivery (VBAC). The American College of Obstetricians and Gynecologists (ACOG) has recently reaffirmed that VBAC is an appropriate alternative to elective repeat cesarean delivery after a single transverse lower segment cesarean delivery.3
In general, the likelihood of VBAC success is approximately 72%.4 Attempted VBAC is associated with a 0.4–0.7% risk of uterine rupture, a risk which is increased with a prior classic or vertical uterine incision, labor induction, and possibly cervical ripening with prostaglandins.4–6 Alternatively, repeated cesarean deliveries are associated with increased risks of placenta accreta, trauma to internal organs, transfusion, hysterectomy, and perioperative complications,7–9 and cesarean delivery is not without risks of fetal injury.10
Considerable effort has been applied to determine factors that alter the likelihood of a successful trial of labor after cesarean delivery and also the maternal and fetal risks associated with attempted VBAC compared with repeat cesarean delivery. In a prospective multi-center observational cohort study of 17,898 women with prior cesarean deliveries attempting a VBAC, we have found an overall VBAC success rate of 73.4% and uterine rupture rate of 0.7% for women with a prior low transverse uterine incision.6 Factors associated with VBAC success included any prior vaginal delivery (odds ratio [OR] 3.9, 95% confidence interval [CI] 3.6–4.3), and the occurrence of one or more prior successful VBAC attempts (OR 2.7, 95% CI 2.2–3.4).11,12 Among women with a single prior cesarean delivery, vaginal delivery before or after the prior cesarean was the strongest predictor of VBAC success after controlling for other factors. Although this study and others have found prior vaginal birth to be associated with increased VBAC success, the impact of prior vaginal delivery on the risk of uterine rupture has been more controversial.13–17 Several studies have suggested that prior VBAC does not alter the risk of uterine rupture and may be associated with increased uterine dehiscence.13–15 Alternatively, in an analysis of women with one or more prior cesarean deliveries, we have found prior VBAC to be associated with a lower uterine rupture risk.18
The relationships between the number of prior VBACs and the probability of successful VBAC attempt or uterine rupture in the current pregnancy remain to be clearly elucidated. It is also unknown if successive labors will place an additive strain on the uterine scar, increasing the risk of uterine rupture when VBAC is attempted. The purpose of this analysis is to evaluate the impact of increasing number of prior VBACs on the likelihood of VBAC success and uterine rupture in subsequent pregnancies.
MATERIALS AND METHODS
This is a secondary analysis of a 4-year observational study 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. Details of the study design have been reported previously.6 Briefly, trained and certified research nurses at each center prospectively identified women who were admitted for delivery and who had a history of cesarean delivery. The medical charts of these women were reviewed for demographic and clinical characteristics, medical and obstetric history, and pregnancy outcomes, including labor characteristics, delivery outcomes, and postpartum complications. Neonatal outcomes, including gestational age at delivery, birth weight, and major morbidities, were collected through hospital discharge or 120 days. Incomplete and equivocal details were resolved through caregiver and patient interview before discharge. Abstracted data were transmitted weekly to the data coordinating center and were edited for missing, out of range, and inconsistent values. Study approval was obtained from the institutional review board at each institution and the George Washington University Biostatistics Center.
For this analysis, we included only women with singleton pregnancies who had previously undergone at least one cesarean delivery by a low transverse uterine incision and who attempted a VBAC and delivered an infant of at least 20 weeks gestation or 500 g in the current pregnancy. Demographic characteristics and clinical outcomes were compared according to the number of prior VBACs subsequent to the last cesarean delivery.
Evaluated pregnancy outcomes included VBAC success, uterine rupture (through and through disruption or tear of the uterine muscle and visceral peritoneum or a uterine muscle separation with extension to adjacent structures), uterine rupture after labor induction, uterine dehiscence (disruption of the uterine muscle with intact serosa), surgical complications (broad ligament hematoma, cystotomy, or bowel or ureteral injury), thromboembolism (deep venous thrombosis or pulmonary embolus), transfusion, endometritis (clinical diagnosis of puerperal uterine infection in the absence of findings suggesting another source), maternal death, umbilical cord arterial pH 7.00 or less, neonatal intensive care unit admission, neonatal hypoxic ischemic encephalopathy, and infant death. All uterine ruptures, maternal deaths, and stillbirths underwent secondary review by local study investigators and final central review to assure accuracy.
Statistical analysis was conducted with SAS 8.2 (SAS Institute, Cary, NC) and StatXact 5 (Cytel Software, Cambridge, MA). To test for trend and to calculate P values among women with increasing number of prior VBACs, the Mantel-Haenszel test was used for categorical variables and the Jonckheere-Terpstra test was used for continuous outcomes.19–20 For one sample binomial interval estimation, the Blyth-Still-Casella interval was used.21 A two-tailed nominal P<.05 was considered significant. No adjustment was made for multiple comparisons.
During the study period, there were 378,063 births, of which 45,988 had a singleton gestation and a history of cesarean delivery. Of those women with a prior cesarean delivery, 13,532 women (29.4%) had a low transverse uterine incision and attempted a VBAC, with an overall success rate of 71.8%. Among those eligible for this analysis, the number of women with 0, 1, 2, 3, and 4 or more prior VBACs were 9,012, 2,900, 1,058, 371, and 191, representing 66.6%, 21.4%, 7.8%, 2.7%, and 1.4% of the cohort, respectively. Outcomes for those with two or more prior VBACs did not change substantially with increasing number, so these women were grouped in one category. Demographic and clinical characteristics for this cohort are presented in Table 1. Maternal age and the frequencies of African-American race and government-assisted health insurance increased with increasing number of prior VBACs. Those with two or more prior VBACs were most likely to have had more than one prior cesarean delivery. Maternal body mass index, the fraction of women with a vaginal delivery before the prior cesarean, and the number of women with spontaneous labor in the current pregnancy did not differ between groups. Delivery gestation and neonatal birth weight declined statistically, but not in a clinically meaningful manner, with increasing number of prior VBACs.
The frequency of VBAC success rose with increasing number of prior VBACs, increasing from 63.3% (95% CI 62.3–64.3%) with no prior VBAC to 87.6% (95% CI 86.4–88.8%) and 90.9% (95% CI 89.5–92.3%) for those with one or two or more prior VBACs, respectively (P<.001, Table 2). The frequency of uterine rupture declined from 0.87% (95% CI 0.68–1.07%) with no prior VBACs to 0.45% (95% CI 0.24–0.76%) and 0.43% (95% CI 0.20–0.85%) for those with one or two or more prior VBACs, respectively (P=.01). The diagnosis of uterine dehiscence was also inversely related to the number of prior VBACs: 0.94% (95% CI 0.75–1.16%) compared with 0.24% (95% CI 0.11–0.48%) and 0.25% (95% CI 0.08–0.61%) for those with 0, 1, or 2 or more prior VBACs, respectively (P<.001). As is demonstrated in Figures 1 and 2, the likelihood of VBAC success appeared to reach a plateau for those with two prior VBACs and did not increase substantially thereafter (P<.001), whereas the risk of uterine rupture declined after one successful VBAC and did not change substantially with additional prior VBACs (P=.03). Further analysis of 12,908 women with only one prior cesarean delivery revealed similar results for pregnancy and neonatal outcomes presented in Table 2 (data not shown).
Other maternal morbidities and peripartum complications were less common with increasing number of prior VBACs, likely related to the decreased need for cesarean delivery in the current pregnancy in this cohort. There were no evident increases in neonatal morbidities or mortality with increasing number of prior VBACs. We found trends toward reductions in the frequency of low 5-minute Apgar scores and hypoxic ischemic encephalopathy, but statistical significance was not reached.
In this analysis, we confirm earlier reports of improved VBAC success with a previous vaginal delivery, including the occurrence of a prior successful VBAC attempt.11–16 In addition, we demonstrate a progressive improvement in the likelihood of VBAC success with increasing number of prior VBACs, with an apparent plateau at a 91% success rate after two prior VBACs. The benefits, risks, and factors affecting the outcomes of attempted VBAC have been well established.4–7,11–16 Although considerable attention has been given to the potential risks and benefits of an attempted VBAC compared with an initial repeat cesarean, there is little information regarding the downstream effects of these decisions on future pregnancies. In a prior publication from this prospectively collected data set, we have demonstrated the risks of repeated cesarean deliveries, including increased risks of placenta accreta, trauma to maternal internal organs, need for hysterectomy and transfusion, and perioperative complications including infectious morbidities.8 Regarding VBAC attempts, we have found that 73.4% of women attempting a VBAC will be successful and 0.7% will have a uterine rupture.6
Whether the risk of uterine rupture is affected by the occurrence of a prior vaginal delivery has not been clear. In separate retrospective studies of 2,204 and 1,216 pregnancies, no differences in the risk of uterine rupture were seen between those with and those without a prior vaginal delivery.14,15 Alternatively, in a third retrospective analysis of 3,783 pregnancies, Zelop et al17 found a lower risk of uterine rupture during a VBAC attempt for those with a prior vaginal delivery than for those without (0.2% compared with 1.1%, P=.01). In a previous analysis of women with one or more prior cesarean deliveries, a history of any prior VBAC was associated with a lower risk of uterine rupture (OR 0.52, 95% CI 0.34–0.82). However, we did not evaluate whether this association was dependent upon the number of prior VBACs.18 In the current analysis, we found women with one or more prior VBACs to have approximately half the risk of uterine rupture when compared with those attempting their first VBAC (0.4–0.5% compared with 0.9%, P=.01). Further, contrary to prior reports of an increased risk of uterine dehiscence with a prior VBAC,14 in this analysis we found the risk of uterine dehiscence to decline with increasing number of VBAC attempts (0.25% compared with 0.9%, P<.001) Thus, our data do not support the notion that successive labors place additive or multiplicative strain on the uterine scar.
A major strength of this study is its size. This cohort of over 13,000 women who attempted VBAC offers insights into outcomes for the overall cohort and also for those with 0, 1, and more than two prior VBACs. Our findings were consistent between the overall cohort and a subanalysis restricted to women with only one prior cesarean delivery. Nevertheless, although the sample size was large, several of the outcomes of interest were uncommon, and the corresponding power to adequately compare outcomes between individual subgroups was limited. Regardless, this prospectively collected data set regarding the benefits and risks of repeated VBAC provides useful estimates of outcomes to practitioners and patients. It is plausible that the evaluation of outcomes in mostly large urban tertiary care hospitals in this study could bias our results toward more favorable outcomes due to increased availability of resources. However, because VBAC should not be attempted in institutions without resources and staffing available for patient monitoring and emergent cesarean delivery, we believe that this, in fact, is not a valid concern and that our findings are generalizable to institutions where VBAC is attempted. Alternatively, the tendency toward more complex patient mix in tertiary care institutions would predispose this population to more complications. Despite this, the vast majority of women attempting a VBAC did not suffer significant complications, regardless of number of prior attempts.
For those who are planning repeated pregnancies after a cesarean delivery, the decision to undergo a repeat cesarean or to attempt a VBAC has downstream consequences. The present analysis demonstrates that, due to the increased probability of VBAC success and decreased probability of uterine rupture with subsequent VBACs, the frequencies of both peripartum and perioperative complications decrease as the number of prior VBACs increases. Those who have had one or more VBACs can be reassured that the likelihood of vaginal delivery is increased and that the likelihood of complications is decreased with respect to the first attempt. This is in contrast to the increasing risks associated with multiple repeat cesarean deliveries.
We believe that the findings of this prospective analysis of a large cohort of pregnancies will provide important information for counseling women who are considering their options regarding VBAC. Our results are particularly important for those considering repeated pregnancies after an initial cesarean delivery. Although women planning large families should consider the risks associated with repeated cesarean deliveries, they should be reassured by the increasing success rates and decreasing risks associated with VBAC attempts in successive pregnancies.
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