A comparative study of obstetric anal sphincter injuries in vaginal deliveries of twins and singleton pregnancies.

AIMS: Increasing numbers of twin pregnancies necessitate investigation of the risk factors associated with obstetric anal sphincter injuries (OASIS). The aims of this study were to establish the incidence of OASIS, and compare women delivering twins to those delivering singletons in risk of OASIS plus maternal, neonatal, and obstetric outcomes.

METHODS: A retrospective cohort study was undertaken and included women delivering in a tertiary London maternity unit between 1999 and 2015; 51 957 eligible women with singleton and 261 with twin pregnancies were identified. Women delivering twins were compared to those delivering singletons in the occurrence of maternal, neonatal, and obstetric outcomes using unadjusted relative risks. A secondary analysis was performed, and conditional logistic regression used to derive an adjusted relative risk of OASIS. Women delivering singletons were matched to those delivering twins for age, parity, ethnicity, gestation, and mode of delivery in a ratio of 2:1.

RESULTS: Compared to the singleton group, mothers delivering twins had significantly more instrumental deliveries (RR [95%CI] 1.92 [1.67-2.27]; P < 0.0001), smaller fetuses (2754.1 vs 3383.8 g; P < 0.001), and were older (32.9 vs 31.0 years; P < 0.0001). The twin group was not significantly different to the singleton group in risk of OASIS (RR 0.61 [0.27-1.33], P = 0.205). The conditional logistic regression demonstrated similar results for the risk of OASIS (adjusted RR = 0.58 [0.22-1.47]; P = 0.253).

CONCLUSIONS: Women delivering twins vaginally were not at a significantly higher risk of sustaining OASIS compared to those delivering singletons.