To validate a previously developed vaginal birth after cesarean (VBAC) prediction model using a patient cohort different than that from which it was derived.We performed a cohort study of all term pregnant women (January 2002-August 2007) with one prior low transverse cesarean delivery attempting a trial of labor. Variables used in the final prediction model (maternal age, prepregnancy body mass index, ethnicity, prior vaginal delivery, prior VBAC, and indication for prior cesarean delivery) were extracted from medical records and used to calculate an individual woman's predicted VBAC success rate. These rates at the level of the study population then were partitioned into deciles and compared with the actual VBAC rates.Of 545 women who fit the inclusion criteria, 502 had complete data available. A total of 262 (52.2%) had VBAC. The predicted probability of VBAC, as calculated by the regression equation, was significantly higher in those who had a successful trial of labor (median 78.4%, interquartile range 62.1-88.2) than in those who did not (median 59.7%, interquartile range 50.8-75.3, P<.001). The predictive model had an area under the receiver operating characteristic of 0.70 (95% confidence interval 0.65-0.74, P<.001), which was similar to that originally described. The actual VBAC rates did not differ from the predicted rates when the predicted chance of success was less than 50%. Above a 50% predicted success, the achieved success rates were consistently 10-20% lower.The published nomogram is predictive of VBAC success. It may help pregnant women contemplating a trial of labor reach a more informed decision.II.