Validation of the Revised Maternal-Fetal Medicine Units Network Vaginal Birth after Cesarean Calculator at an Academic Community Hospital

Objective The 2007 Maternal-Fetal Medicine Units Network vaginal birth after cesarean (VBAC) calculator included race and ethnicity among the score criteria. These variables were removed in 2021 to decrease the risk of systemic biases impacting counseling regarding birth after cesarean delivery (CD). This study compares test performance of each version of the calculator within our population. Study design Retrospective cohort study of pregnant patients ≥18 years old eligible for TOLAC after one CD between September 2019 and August 2022. Predicted VBAC success scores were calculated for each patient using both the 2007 and 2021 calculators. Predicted scores were separated into categories and patients were stratified by self-identified race/ethnicity for comparison. Receiver operating characteristic (ROC) curves were created to compare sensitivity and specificity of each calculator using predicted scores as continuous variables and as categories (>50%, >60%, and >70%) with actual VBAC outcomes. Calibration metrics were also calculated for each calculator. Results 457 patients met inclusion criteria. Race/ethnicity distribution was 55.0% White, 8.1% Black, 33.7% Hispanic. VBAC success rate was 76.4%. A higher proportion of Black patients had lower predicted scores with the 2007 calculator vs. the 2021 calculator. ROC curves showed similar areas under the curve for both calculators (AUC 0.74 2007 vs. AUC 0.75 2021; p=.22), consistent with similar predictive values between both calculators. The revised calculator had better agreement between observed and predicted VBAC rates within deciles of predicted probabilities. Conclusions In our population, predicted VBAC success rates were lower among Black patients with the 2007 calculator. Both calculators performed similarly in predicting VBAC success with the new calculator having better agreement within deciles of predicted probability. Our data contribute to supporting the VBAC calculator revision, which removed race and ethnicity as criteria in obstetric management of pregnant patients with one CD desiring to attempt TOLAC.