External validation and comparison of the 2007 and 2021 Grobman models for predicting successful vaginal birth after cesarean: A retrospective cohort study.

OBJECTIVE

This study externally validated and compared the performance of the antepartum 2007 and 2021 Grobman models for predicting successful vaginal birth after cesarean (VBAC) in a Vietnamese cohort.

METHODS

This retrospective cohort study included pregnant women with a single prior cesarean section, a term pregnancy, and fetal cephalic presentation, enrolled at Hue University of Medicine and Pharmacy Hospital between October 2023 and June 2025. Data from participants with a history of a single low-transverse cesarean section were collected for both models to predict VBAC success. Demographic and clinical data were collected, focusing on maternal age, body mass index (BMI), and delivery history. Statistical analyses included receiver operating characteristic (ROC) curves, calibration curves, and decision curve analysis (DCA) to evaluate model performance.

RESULTS

Among the 147 participants, 65 (44.22%) had a successful VBAC. The two models showed comparable discrimination: the 2021 model yielded a numerically higher AUC (83.24%) than the 2007 model (79.83%), but the difference was not statistically significant (DeLong P = 0.115). At the operating points selected by the Youden index, sensitivity was similar (72.31% vs. 70.77%), whereas the 2021 model showed a numerically higher specificity (91.46% vs. 76.83%). The 2021 model also showed numerically higher positive predictive value and a lower Brier score, although both models were miscalibrated. DCA showed a numerically higher net benefit for the 2021 model across the examined threshold range.

CONCLUSION

The 2021 race-neutral Grobman model demonstrated discrimination comparable to the 2007 version, with a numerically higher specificity at a data-derived operating point and numerically closer calibration; these differences are descriptive rather than confirmed statistically, and both models require recalibration before routine use.