Risk factors for obstetric anal sphincter injuries in twin deliveries: a retrospective review

Introduction and hypothesis: Risk factors for obstetric anal sphincter injuries (OASIS) have been well-established in singleton pregnancies. Considering the unique characteristics of twin deliveries, our aim was to identify risk factors for OASIS that are specific to twins. Methods: A retrospective study of all vaginal twin deliveries in a tertiary center between 2000 and 2014. Women who experienced OASIS (the OASIS group) were compared with those whose anal sphincter was intact (controls). Results: Overall 717 women were eligible for the study, of whom 20 (2.8 %) experienced OASIS. Women in the OASIS group were more likely to be nulliparous (95.0 % vs 53.7 %, p < 0.001) and were characterized by a higher gestational age at delivery (36.1 ± 2.5 vs 34.6 ± 3.3, p = 0.04), a higher birth weight for both twin A and twin B (2,507 ± 540 g vs 2,254 ± 525 g, p = 0.03, and 25,49 ± 420 g vs 2,232 ± 606 g, p = 0.004 respectively), and a higher rate of episiotomy (40.0 % vs 14.2 %, p = 0.001), instrumental delivery for twin A (80.0 % vs 13.5 %, p < 0.001) or twin B (80.0 % vs 18.7 %, p < 0.001), and inter-twin delivery interval of over 30 min (20.0 % vs 7.5 %, p = 0.04). The only factor that remained significant on multivariate analysis was instrumental delivery: forceps delivery of twin A (OR = 8.8, 95 % CI 2.6–30.1), vacuum extraction of twin A (OR = 9.2, 95 % CI 2.6–34.6), and forceps delivery of twin B (OR = 15.4, 95 % CI 4.9–48.6). In women with certain combinations of risk factors the risk of OASIS was as high as 30 %. Conclusion: The overall rate of OASIS in twins is low and instrumental delivery, especially by forceps, is a risk factor.