Does birthweight have a role in the effect of episiotomy on anal sphincter injury?

Purpose Most professional guidelines advise against routine episiotomy during vaginal delivery, although mounting evidence supports its protective role regarding obstetric anal sphincter injury (OASI). We aimed to study the effect of lateral and mediolateral episiotomies on the rate of OASI in relation to birthweight among nulliparous women undergoing vaginal delivery. Methods A historical cohort study was conducted of all nulliparous women who delivered vaginally at term between 2011 and 2019 at a tertiary university hospital. Women were allocated into two groups: (1) with OASI and (2) without OASI. Episiotomy performance and birthweight groups were analyzed. Results Overall, 22,250 deliveries were analyzed for inclusion: 18,533 (83.3%) spontaneous vaginal deliveries (SVD), 3222 (14.5%) vacuum-assisted deliveries (VAD) and 495 (2.2%) forceps deliveries. Total episiotomy and OASI rate was 48.2% and 1.7%, respectively. Episiotomy rate was lower in the OASI group as compared to the no OASI group (158 (41.3%) vs. 10,568 (48.3%), OR 0.75, 0.61–0.92, p = 0.006). Median birthweight was higher for OASI group neonates (3355 vs. 3160, p 4000 g (OR 0.56, 0.38–0.82, p = 0.003; 0.66, 0.45–0.99, p = 0.04 and 0.24, 0.07–0.78, p = 0.01, respectively). In VADs, episiotomy decreased the rate of OASI in the neonatal weight groups of 2500–2999 and 3000–3499 g (OR 0.36, 0.14–0.89, p = 0.02 and OR 0.38, 0.19–0.75, p = 0.004, respectively). Conclusions Lateral and mediolateral episiotomies are independent modifiable predictors of OASI, protective against OASI in SVDs when neonates weigh > 3000 g and 2500–3499 g in VADs.