Association between second stage of labour length and risk of obstetrical anal sphincter injury in nulliparous women: a population-based retrospective cohort study

Introduction and hypothesis Obstetrical anal sphincter injury (OASIS) is a common consequence of vaginal delivery in nulliparas and carries the risk of short- and long-term morbidity. The objective of this study was to estimate the association between the duration of the second stage of labour and OASIS risk. Methods A population-based, retrospective cohort of nulliparas delivering singleton, vertex, non-anomalous fetuses at term in Nova Scotia, Canada, from 2005 to 2019, were identified using the Nova Scotia Atlee Perinatal Database. Poisson regression models were used to estimate risk ratios (RR) with robust 95% confidence intervals (CI) adjusting for confounding variables to investigate the association between the length of the second stage and OASIS in the entire cohort and in operative vaginal deliveries. Results Of 36,662 participants, 7.6% sustained an OASIS (6.8% third-degree, 0.8% fourth-degree tear). The proportion of participants who sustained an OASIS increased over the study period. For each 30-min increase in the length of second stage, the OASIS risk increased by 11% (RR 1.11, 95% CI 1.10–1.12). When stratified by mode of delivery, second stage length ≥ 90 min was associated with an increased OASIS risk in spontaneous (RR 1.35, 95% CI 1.15–1.58) and vacuum-assisted vaginal deliveries (RR 1.42, 95% CI 1.11–1.81). In forceps-assisted vaginal deliveries, OASIS risk was increased, with shorter and longer durations of the second stage. Conclusion Increasing length of the second stage of labour was associated with increasing risk of OASIS overall, but the association was heterogeneous between modes of delivery. Length of the second stage should be considered in counseling about OASIS risk.