Srinivas, Sindhu K. MD1; Stamilio, David M. MD, MSCE1; Stevens, Erika J. MA2; Odibo, Anthony O. MD2; Peipert, Jeffrey F. MD2; Macones, George A. MD, MSCE2
Approximately 26% of the 4 million births per year in the United States are cesarean deliveries, and this number will only continue to grow as the role of elective primary cesarean delivery is further expanded.1 Over the past 2 decades, obstetricians have been encouraged to offer women with a previous low transverse cesarean an attempt at vaginal birth. Past research has indicated that a strategy of vaginal birth after cesarean delivery (VBAC) has a reasonable success rate—as high as 60–80% in the overall VBAC population.2 However, these success rates differ based on clinical factors. More recent research has focused on maternal and neonatal risks associated with VBAC, as well as predictors of VBAC success and failure.3–5 Complication rates appear to be greatest in those who fail a trial of labor and subsequently require a cesarean delivery.6 Conversely, safety is greatest in those who attempt a trial of labor after a prior cesarean delivery and are successful.
Several studies have found that many factors are associated with VBAC failure, such as preeclampsia, macrosomia, maternal obesity, and labor induction.3,4,7 Some have proposed different scoring mechanisms to incorporate these individual factors and attempt to reliably predict VBAC success, failure, or complications. Troyer and Parisi,8 Vinueza et al,9 and Gonen and colleagues10 have all attempted to develop different scoring systems to predict VBAC success. However, these systems were all developed using a relatively small number of patients and do not incorporate several factors that have been associated with VBAC success. In general, prior attempts to develop a means of predicting a successful VBAC trial have not been successful.6–8
Our aim was to identify a group of clinical factors, both prelabor and intrapartum, in a large multicenter cohort that could be used to accurately predict failure of achieving a vaginal delivery in women attempting VBAC.
MATERIALS AND METHODS
We conducted a planned secondary analysis within a retrospective cohort study of 25,005 women with prior cesarean deliveries from 1996 to 2000. The details of the study methods have been published previously,11,12 but a brief description follows. Of the 17 northeastern United States hospitals included in the study, six were tertiary care centers with residency programs, five were community-based hospitals with residency programs, and six were nonteaching community hospitals. Individual hospital names are available upon request. All local institutional review boards approved the conduct of the study. The cohort of patients was identified via International Classification of Diseases, 9th Revision (ICD-9) codes for “prior cesarean delivery, delivered.” All charts for eligible patients were sequentially studied. We excluded patients with unknown prior cesarean scar type or prior classic uterine incision. There were 478 patients excluded for a prior vertical cesarean, and of these, 460 were termed classic and 18 just vertical in the medical record. Trained research nurse abstractors collected demographic information, medical and obstetric history, and data on complications, treatment, and outcome of the index pregnancy using standardized, closed-ended, data collection forms. To assure accurate and valid data collection, training for abstractors was repeated at various predetermined points during the study and when quality assurance measures (repeat abstraction of 3%) indicated the need.
In the original retrospective cohort study, the primary delivery-related adverse outcome was uterine rupture. This outcome was identified by the data abstractors using strictly defined diagnostic criteria that have been previously described.11,12 Secondary adverse maternal outcomes included in the initial cohort study were operative complications such as bladder, ureter, or bowel injury and uterine artery laceration.
For this planned secondary analysis, we identified VBAC failure as the primary outcome of interest and defined it as the inability to achieve a vaginal delivery with a VBAC attempt. We used univariable and multivariable (ie, logistic regression) statistical methods to develop a multivariable prediction model for the outcome of VBAC failure. We selected potential predictors for inclusion in the final regression model based on the unadjusted analysis (P<.2) and proven or suspected biological importance. The final regression model was developed by sequentially removing covariates and testing differences in hierarchical models with the likelihood ratio test; the final model includes significant variables identified to be the most predictive of VBAC failure. We also assessed hospital as a confounder in this analysis and analyzed the possibility of clustering effects by hospital in prior analyses (Srinivas SK, Stamilio DM, Sammel MD, Stevens EJ, Peipert JF Odibo AO, Macones GA. Vaginal birth after cesarean delivery: does maternal age affect safety and success? Paediatr Perinat Epidemiol [in press]). The entire analysis was performed with Stata 8 Special Edition (StataCorp, College Station, TX).
The total number of patients included in the cohort was 25,005. Of these, 13,706 (54.81%) attempted vaginal birth after cesarean delivery, and 11,299 (45.19%) elected repeat cesarean. Of those who attempted VBAC, there was a 24.5% failure rate.
The demographics of the entire cohort, as illustrated in Table 1, are representative of a general obstetric population. The women who attempted a vaginal birth were similar to those who elected a cesarean delivery with respect to several factors, such as age, gestational age at delivery, and birth weight more than 4,000 g. In the VBAC and elective cesarean groups, roughly two thirds of women were aged 21–34 years. The rates of preterm birth in the VBAC and elective cesarean groups were similar to each other and to the national rate. Health care third party payor types were evenly distributed and similar between the VBAC and elective cesarean groups. However, there were more African-American women and women obtaining care at a university hospital in the VBAC attempt group.
The unadjusted analysis is displayed in Table 2. Several factors, including gestational age, the presence of diabetes or chronic hypertension, birth weight, labor type, prior indication for cesarean delivery, and prior vaginal delivery, were all significantly associated, either positively or negatively, with failure of a VBAC attempt.
Table 3 exhibits the results of the multivariable logistic regression analysis for the outcome, failed VBAC. The final model consists of six variables, including maternal race and age, gestational age at delivery, spontaneous (versus induced) labor, prior cesarean indication of cephalopelvic disproportion or failed induction, and history of vaginal delivery. The variables diabetes, chronic hypertension, and birth weight more than 4,000 g were not included in the final predictive model because, although they were associated with VBAC failure, they did not improve predictive accuracy and were felt to be biologically less important for prediction of VBAC failure because of modest effect size or prevalence or both. Having a prior vaginal delivery was the most protective against a failed VBAC attempt (odds ratio 0.21, 95% confidence interval 0.19–0.24), and labor induction had the largest magnitude of association with VBAC failure. Figure 1 illustrates the receiver operating characteristics curve for our prediction model. The area under the receiver operating characteristics curve is 0.7170. This model for VBAC failure prediction has a sensitivity of 11.08%, a specificity of 96.64%, a positive predictive value of 52.17%, and a negative predictive value of 76.66%. The receiver operating characteristics curve reveals an unfavorable trade-off between sensitivity and false-positive rate for this prediction model. For example, selecting an optimal cutoff point on the curve to achieve 75% sensitivity for VBAC failure results in a false-positive rate of about 40%.
When evaluating subgroups of women by using the two strongest clinical factors, namely, labor type and prior vaginal delivery history, the unadjusted rate of VBAC success was highest for women with a prior vaginal delivery and spontaneous labor (Table 4). Conversely, women without a prior vaginal delivery who underwent labor induction had the lowest VBAC success rate. Table 4 summarizes the adjusted odds of VBAC failure and observed rates of VBAC success for particular subsets of women using the two strongest clinical risk factors.
In our large retrospective cohort, VBAC failure cannot be predicted efficiently by using a combination of the strongest clinical risk factors. However, there appear to be strong factors that are highly associated with VBAC failure (ie, labor induction) and VBAC success (ie, prior vaginal delivery). The presence or absence of these factors may help in counseling patients about attempting a trial of labor.
With the rise in cesarean delivery rate, there has been a growing interest in identifying long-term or “down-stream” reproductive health consequences of a continually increasing number of cesarean deliveries. Concerns include increased maternal operative complications and adverse neonatal outcomes, particularly in patients with multiple prior cesareans and in patients who require cesarean delivery after a trial of labor. In an effort to better target therapy and minimize further inflation of the rates of cesarean delivery and adverse perinatal and maternal outcome, clinical researchers have sought reliable predictors of VBAC success.
Maternal obesity, pregestational diabetes, increasing number of prior cesarean deliveries, fetal macrosomia, and having had a prior cesarean for cephalopelvic disproportion decrease the chance of VBAC success, whereas nonrecurring or nonpersistent indications for cesarean (ie, breech presentation and nonreassuring fetal heart tracing) are associated with an increased likelihood of success.3,5,6,13 Having had a prior vaginal delivery is strongly associated with increased VBAC success.14 Our results corroborate this finding.
The study design used for this investigation, a multicenter retrospective cohort, has several important advantages. First, the data set is robust, with information on clinically relevant exposure, outcome, and confounding variables that were strictly defined with a minimal amount of missing data. Second, the results are more generalizable to many obstetric populations because subjects were selected from various types of hospitals. Third, the rates of uterine rupture and other adverse VBAC-related outcomes in our cohort corroborate those estimated in other studies, imparting validity to our results.2,15–18
Although this study has many attributes, there are some limitations. First, we were restricted to the variables contained in the data set. Although the data set is fairly comprehensive, some covariates are not available. For example, we had no data on maternal weight, body mass index, weight gain during pregnancy, or detailed characteristics of labor induction techniques. These variables may be biologically important predictors of VBAC failure, especially maternal obesity. However, given the previously observed modest magnitude of effect of obesity on vaginal delivery rate, it is unlikely that this one factor would appreciably improve our prediction model. Additionally, we had no information on neonatal outcomes because this study was primarily established to assess maternal outcomes of VBAC. Second, if the clinicians had preconceptions regarding VBAC and success that systematically affected their counseling techniques, there is the potential for confounding by indication.
This study specifically evaluates the effect of various clinical factors (prelabor and labor) on VBAC failure and attempts to develop a prediction rule for failed VBAC. These data validate prior studies that identified several risk factors associated with VBAC failure and protective factors associated with success. Despite our large multicenter cohort and rigorous analysis, we were unable to develop a single reliable clinical prediction rule for VBAC failure that can be applied to all women. However, we were able to identify clinically helpful information for VBAC counseling using the two strongest clinical risk factors, prior vaginal delivery and labor type. Obstetricians can modify their counseling about trial of labor using the knowledge about known factors associated with VBAC failure and success found in our analysis.
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© 2007 by The American College of Obstetricians and Gynecologists.