Prediction of Obstetric Anal Sphincter Injury in Nulliparous Women: Model Development and Temporal Validation

OBJECTIVES: Development and validation of two prediction models for obstetric anal sphincter injury (OASI). DESIGN: Population-based cohort study. SETTING: Nationwide (the Netherlands). POPULATION: Data from the Netherlands Perinatal Registry, describing nulliparous women who delivered a singleton live born infant in cephalic presentation at term from 2016 to 2020, with spontaneous (SVD) or operative vaginal delivery (OVD). METHODS: Based on literature and clinical expertise, a set of potential predictors was defined and derived from the national perinatal registry. A predictive model was constructed, and accessible nomograms provided. Internal and temporal external validation was performed. MAIN OUTCOME MEASURES: OASI rate. RESULTS: The risk of OASI in 171 046 women with SVD was 4.1%. After logistic regression with step-wise backward selection using Akaike Information Criterion (AIC), ten predictors were retained. These were: mediolateral episiotomy (MLE), expected fetal birth weight, duration of the 2nd stage, occipitoposterior presentation, induction of labour, epidural analgesia, Asian ethnicity, maternal age, gestational age and fetal sex. The final model had a moderate discriminative ability (AUC 0.67, 95% CI 0.67-0.68) and excellent calibration (Brier score 0.039). The average risk of OASI in 37 547 women with OVD was 3.5%. Seven predictors were retained in the model: MLE, expected fetal birth weight, duration 2nd stage of labour, occipitoposterior fetal presentation, epidural analgesia, Asian ethnicity and gestational age. The final model had moderate discrimination (AUC 0.68, 95% CI 0.67-0.70) and excellent calibration (Brier score 0.032). CONCLUSIONS: A prediction model for OASI was developed and validated for both nulliparous women with spontaneous vaginal delivery and with operative vaginal delivery. These models can form a basis to identify women with a high risk of OASI.