Simplifying the prediction of vaginal birth after cesarean delivery: role of the cervical exam

Objective: Predicting likelihood of vaginal birth after cesarean (VBAC) is a cornerstone in counseling patients considering a trial of labor after cesarean (TOLAC). Yet, the simplified Bishop score (SBS), a score comprised cervical dilation, station, and effacement assessment used to predict successful vaginal delivery, has not been applied to the TOLAC population. We evaluated the relationship between admission SBS and likelihood of successful VBAC. We also determined the predictive characteristics of SBS, compared to cervical dilation alone, for successful VBAC. Methods: This is a secondary analysis of a prospective cohort study of patients with a singleton gestation, ≥37 0/7 weeks gestation, and prior cesarean admitted to Labor & Delivery between 2010 and 2014. The primary outcome of successful VBAC was compared between those with a favorable (score >5) and unfavorable (score ≤5) admission SBS. Secondary outcomes were select maternal and neonatal outcomes. Adjusted risk ratios were estimated using multivariable logistic regression analyses. Receiver-operating characteristic curves compared predictive capabilities of cervical dilation alone to SBS for successful VBAC. Results: Of the 656 patients who underwent a TOLAC during the study period, 421 (64%) had a successful VBAC. 203 (31%) and 453 (69%) had a favorable and an unfavorable admission SBS, respectively. After adjusting for body mass index and prior vaginal delivery, patients with a favorable admission SBS had a 30% greater likelihood of successful VBAC compared to those with an unfavorable SBS (aRR 1.30, 95% CI 1.16–1.40). Admission cervical dilation alone performed similarly to SBS as a predictor of successful VBAC, with a receiver-operator characteristic curve area under the curve (AUC) of 0.68 (95% CI 0.64–0.72) versus an AUC 0.66 (95% CI 0.62–0.70), respectively (p =.07). There were no differences in adverse maternal or neonatal outcomes between those with an unfavorable and favorable SBS. Conclusions: A favorable admission SBS is associated with an increased likelihood of VBAC. Although both admission SBS and cervical dilation alone are only modest predictors of VBAC, admission cervical dilation performs overall similarly to current models for VBAC prediction and is an objective, reproducible, and generalizable measure. Our study highlights the value of waiting until end of pregnancy (rather than the first prenatal visit) to conclude patient counseling on the decision to TOLAC in order to consider admission cervical assessment, particularly cervical dilation.