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To establish the contribution of maternal, fetal and intrapartum factors to the risk of incidence of obstetric anal sphincter injuries (OASIS) and assess the feasibility of an OASIS risk prediction model based on variables available to clinicians prior to birth.

This was a population-based, retrospective cohort study using single-site data from the birth database of Aarhus University Hospital, Denmark. The participants were all women who had a singleton vaginal birth during the period 1989 to 2006. Univariate and multivariate logistic regression analyses were performed using multiple imputations for missing data and internally validated using bootstrap methods. The main outcome measures were the contributions of maternal, fetal and intrapartum events to the incidence of OASIS.

A total of 71,469 women met the inclusion criteria, of whom 1,754 (2.45 %) sustained OASIS. In the multivariate analysis of variables known prior to birth, maternal age 20 - 30 years (OR 1.65, 95 % CI 1.44 - 1.89) and ae30 years (OR 1.60, 95 % CI 1.39 - 1.85), occipitoposterior fetal position (OR 1.34, 95 % CI 1.06 - 1.70), induction/augmentation of labour (OR 1.46, 95 % CI 1.32 - 1.62), and suspected macrosomia (OR 2.20, 95 % CI 1.97 - 2.45) were independent significant predictors of OASIS, with increasing parity conferring a significant protective effect. The ’prebirth variable’ model showed a 95 % sensitivity and a 24 % specificity in predicting OASIS with 1 % probability, and a 3 % sensitivity and a 99 % specificity in predicting OASIS with a 10 % probability.

Our model identified several significant OASIS risk factors that are known prior to actual birth. The prognostic model shows potential for ruling out OASIS (high sensitivity with a low risk cut-off value), but is not useful for ruling in the event.