characteristics and demographic information were collected, as well as labor characteristics. Onset of labor data was available for 63% of the study population. The main exposure variable was duration of labor, based on the time of onset of established contractions as reported by the patient or midwife and the time of birth. Induction of labor, augmentation of labor, and epidural analgesia were all secondary exposure variables. Poisson regression was used to calculate risk estimates for uterine rupture according to exposure variables.Among the study population of 20,046 patients undergoing a trial of labor following a single CD, 282 (1.4%) experienced uterine rupture. Uterine rupture more commonly occurred in older and shorter patients, and patients who required an interpreter during pregnancy. Of the 282 uterine ruptures, only 161 (63%) had data for onset of regular contractions reported. Among the 121 cases with no labor duration information, the incidence of uterine rupture was 1.6%. Estimates were presented as adjusted rate ratios (ARR) with 95% confidence intervals (CI). The mean duration of labor among patients with uterine rupture was 9.88 hours [95% CI, 8.93-10.83], compared with 8.20 hours (95% CI, 8.10-8.31) among patients who delivered vaginally, and 10.71 hours (95% CI, 10.46-10.97) among patients who delivered via intrapartum cesarean section (CS) without rupture. Uterine rupture in the first 3 hours of labor occurred in 1.0/1000 births, and in the first 12 hours of labor, 15.6/1000. At 6 hours of labor, patients had the highest risk for uterine rupture compared with vaginal delivery (