Risk factors related to obstetric anal sphincter injuries (OASIS) are known for singleton deliveries. No study to date has described the rate and risk factors involved in twin deliveries.We conducted a retrospective cohort study (1985-2010) of all vaginal twin and singleton deliveries in a single tertiary center. The adjusted odds ratio (OR) for OASIS at delivery was estimated using the logistic regression model.The study comprised 1,538 and 91,312 patients with vaginal twin and singleton delivery, respectively. Twenty twin (1.27?%) and 2,331 (2.55?%) singleton deliveries were complicated with OASIS. The following OASIS-associated risk factors were shared by both populations: nulliparity [twins adjusted OR 5.9, 95?% confidence interval (CI) 1.7-20.9; singletons adjusted OR 3.9, 95?% CI 3.5-4.4), occipitoposterior (OP) position (twins adjusted OR 3.00, 95?% CI 1.1-8.0; singletons adjusted OR 1.6, 95?% CI 1.3-2.00), birthweight (for each 100?g) (twin adjusted OR 1.1, 95?% CI 1.0-1.2; singletons adjusted OR 1.07, 95?% CI 1.06-1.08), and instrumental delivery (twins adjusted OR 4.3, 95?% CI 1.2-15.4, singletons adjusted OR 2.4, 95?% CI 2.2-2.6).Risk factors of nulliparity, OP position, large fetal size, and instrumental delivery were shared by both twin and singleton deliveries. These data will be useful in counselling women carrying twins who intend to deliver vaginally.
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