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To characterize probabilities of vaginal delivery based on second-stage duration along with maternal and neonatal risks for women undergoing labor after cesarean delivery.This unplanned secondary analysis of the Maternal-Fetal Medicine Units Cesarean Registry, a prospective observational cohort, assessed outcomes in women with a prior uterine scar and included women with a previous cesarean delivery without prior vaginal delivery who reached the second stage of labor. The primary outcome was mode of delivery by second-stage duration. Secondary outcomes included assessment of individual adverse maternal (chorioamnionitis, atony, endometritis, hysterectomy, uterine rupture or dehiscence, and red cell transfusion) and neonatal (cord pH less than 7.10, Apgar score less than 6 at 5 minutes, neonatal intensive care unit admission, and ventilatory support) outcomes.Of 4,579 women with a previous cesarean delivery who reached the second stage of labor, 4,147 (90.6%) delivered vaginally. As second stage increased, successful vaginal delivery rates decreased: 97.3% at less than 1 hour (95% CI 96.6–97.9%), 91.5% at 1 to less than 2 hours (95% CI 89.8–93.1%), 78.5% at 2 to less than 3 hours (95% CI 74.5–82.1%), 62.3% at 3 to less than 4 hours (95% CI 55.2–69.1%), and 45.6% at 4 hours or greater (95% CI 37.7–53.7%). Risk of all adverse maternal outcomes increased with the length of the second stage. Specifically, risk of uterine rupture or dehiscence increased with second-stage length from less than 1 hour (0.7%), 1 to less than 2 hours (1.4%), 2 to less than 3 hours (1.5%), to 3 hours or greater (3.1%) (P<.001 for differential risk across the second stage). Risk of neonatal outcomes did not differ significantly by second-stage length.Although many women with a longer second stage (greater than 3 hours) will achieve successful vaginal delivery, these patients may be at increased risk for adverse maternal outcomes and should have close observation of fetal heart rate monitoring, maternal vital signs, and symptoms suggestive of uterine rupture or dehiscence.