Is the risk of obstetric anal sphincter injuries increased in vaginal twin deliveries?

Objective: Vaginal twin deliveries have a higher rate of intrapartum interventions. We aimed to determine whether these characteristics are associated with an increased rate of obstetric anal sphincter injuries compared with singleton.Study design: Retrospective study of all twin pregnancies undergoing vaginal delivery trial was conducted from January 2000-September 2014. Sphincter injury rate compared with all concurrent singleton vaginal deliveries. Multivariable analysis was used to determine twin delivery association with sphincter injuries while adjusting for confounders.Results: About 717 eligible twin deliveries. Outcome was compared with 33 886 singleton deliveries. Twin pregnancies characterized by a higher rate of nulliparity (54.8% versus 49.5%, p = 0.005), labor induction (42.7% versus 29.1%, p < 0.001), and instrumental deliveries (27.5% versus 16.7%, p < 0.001), lower gestational (34.6 ± 3.3 versus 38.8 ± 2.3, p < 0.001), and lower birth weight. Total breech extraction was performed in 29.0% (208/717) of twin deliveries. Overall obstetric sphincter injury rate was significantly lower in the twins group (2.8% versus 4.4%, p = 0.03, OR = 0.6, 95% CI 0.4-0.9), due to lower rate of 3rd degree tears in twins versus singletons (2.2% versus 4.0%, p = 0.02), rate of 4th degree tears similar among the groups (0.6% versus 0.4%, p = 0.5). In multivariable analysis, sphincter injuries were associated with nulliparity (OR = 3.9, 95% CI 3.4-4.5), forceps (OR = 6.8, 95% CI 5.8-7.8), vacuum (OR = 2.9, 95% CI 2.5-3.3), earlier gestational age (OR = 0.2, 95% CI 0.1-0.3), episiotomy (OR = 0.8, 95% CI 0.7-0.9), and birth weight over 3500 g (OR = 1.8, 95% CI 1.6-2.0). However, the association between twins (versus singletons) deliveries and sphincter injuries was lost after adjustment for delivery gestational age (OR = 0.7, 95% CI 0.4-1.2).Conclusion: Despite a higher rate of intrapartum interventions, the rate of sphincter injuries is lower in twins versus singleton deliveries, mainly due to a lower gestational age at delivery.