Time matters – a Swedish cohort study of labor duration and risk of uterine rupture

Introduction Uterine rupture is an obstetric emergency associated with maternal and neonatal morbidity. The main risk factor is a prior cesarean section, with rupture occurring in subsequent labor. The aim of this study was to assess the risk of uterine rupture by labor duration and labor management. Material and methods This is a Swedish register-based cohort study of women who underwent labor in 2013-2018 after a primary cesarean section (n = 20 046). Duration of labor was the main exposure, calculated from onset of regular labor contractions and birth; both timepoints were retrieved from electronic medical records for 12 583 labors, 63% of the study population. Uterine rupture was calculated as events per 1000 births at different timepoints during labor. Risk estimates for uterine rupture by labor duration, induction of labor, use of oxytocin and epidural analgesia were calculated using Poisson regression, adjusted for maternal and birth characteristics. Estimates were presented as adjusted rate ratios (ARR) with 95% confidence intervals (CI). Results The prevalence of uterine rupture was 1.4% (282/20 046 deliveries). Labor duration was 9.88 hours (95% CI 8.93-10.83) for women with uterine rupture, 8.20 hours (95% CI 8.10-8.31) for women with vaginal delivery, and 10.71 hours (95% CI 10.46-10.97) for women with cesarean section without uterine rupture. Few women (1.0/1000) experienced uterine rupture during the first 3 hours of labor. Uterine rupture occurred in 15.6/1000 births with labor duration over 12 hours. The highest risk for uterine rupture per hour compared with vaginal delivery was observed at 6 hours (ARR 1.20, 95% CI 1.11-1.30). Induction of labor was associated with uterine rupture (ARR 1.54, 95% CI 1.19-1.99), with a particular high risk seen in those induced with prostaglandins and no risk observed with cervical catheter (ARR 1.19, 95% CI 0.83-1.71). Labor augmentation with oxytocin (ARR 1.60, 95% CI 1.25-2.05) and epidural analgesia (ARR 1.63, 95% CI 1.27-2.10) were also associated with uterine rupture. Conclusions Labor duration is an independent factor for uterine rupture among women attempting vaginal delivery after cesarean section. Medical induction and augmentation of labor increase the risk, regardless of maternal and birth characteristics.