RESUMEN

INTRODUCTION: The Maternal-Fetal Medicine Units Network calculator for vaginal birth after cesarean (VBAC) probability was updated to exclude a race and ethnicity variable, but its impact on marginalized groups is unknown. We assessed the tool with attention to birth history and body mass index (BMI) in a predominately Hispanic cohort and examined the possible effect of discouraging labor after cesarean (LAC) with a low score. METHODS: We conducted a retrospective cohort study of LACs by patients who entered spontaneous or induced labor with a live, nonanomalous fetus in cephalic presentation between 2012 and 2022. Observed VBAC rates were compared with the mean predicted VBAC probability derived from the calculator. Areas under the curve (AUCs) were calculated for the entire cohort and for individuals with LAC with and without prior vaginal birth. A z-test was used to determine the significance between VBAC rates in 4 BMI categories. The impact of discouraging LAC with VBAC probability below 70% was examined. RESULTS: A total of 400 people experienced 507 LACs, with 417 (82.2%) resulting in VBAC compared with a mean predicted probability of 71.2%. The AUC for all LACs was 0.76 (95% CI, 0.71-0.81), whereas the AUC for LACs with prior vaginal birth was 0.70 (95% CI, 0.56-0.85) and without was 0.60 (95% CI, 0.52-0.67). Observed VBAC rates exceeded predicted rates for individuals with overweight and obese BMIs: <25 (79.6% vs 75.6%; P = .520), 25 to 30 (83.2% vs 71.9%; P = .007), 30 to 40 (82.7% vs 70.0%; P = .004), and ≥40 (82.8% vs 58.3%; P = .040). Discouraging LAC below 70% probability might have prevented 71 unsuccessful LACs, 160 initial VBACs, and 57 subsequent VBACs, decreasing the VBAC rate to 39.4% (200/507). DISCUSSION: In a predominately Hispanic cohort, the updated calculator underestimated VBAC potential for people with no vaginal birth history and/or an elevated BMI. Discouraging LAC based on low VBAC probability may prevent both initial and subsequent VBACs.