A dynamic prediction score for obstetric anal sphincter injury among nulliparous women delivering vaginally

Objective: To develop a risk score for obstetric anal sphincter injury (OASI) occurrence among nulliparous women delivering vaginally, based on data available at admission for delivery and as labor progresses. Methods: A retrospective study of all nulliparous women who delivered vaginally between March 2011 and January 2021. Characteristics were compared between OASI and no-OASI groups. Multivariable analyses were performed to identify independent risk factors for OASI occurrence. Results: OASI occurred in 453 (1.7%) of 26 081 women who met the inclusion criteria. The following variables were independently associated with OASI: maternal height (adjusted odds ratio [aOR] 0.97, 95% confidence interval (CI) 0.95–0.99), hypertensive disorders (aOR 1.74, 95% CI 1.03–2.95), sonographic fetal weight estimation (aOR 1.00, 95% CI 1.00–1.00), second-stage duration (aOR 1.00, 95% CI 1.00–1.00), occiput posterior position (aOR 2.87, 95% CI 1.79–4.62), and episiotomy performance (aOR 0.63, 95% CI 0.47–0.84). In a risk score based on variables available at admission for delivery, the presence of two factors was associated with 4.3% OASI risk. Upon incorporating intrapartum variables, the presence of two risk factors was associated with 2.9% OASI rate. Conclusion: A dynamic risk score for OASI occurrence based on data available at admission for delivery and as delivery progresses can assist in counseling regarding OASI risk. A dynamic risk score for obstetric anal sphincter injury occurrence based on data available at admission for delivery and intrapartum was developed.