Purpose: Nulliparity and operative vaginal delivery are established risk factor for obstetric anal sphincter injury (OASI). However, risk factors for OASIS occurrence among parous women delivering vaginally are not well-established. We aimed to study the risk factors for OASI occurrence among parous women.
Methods: A retrospective study including all parous women who delivered vaginally at term during 2011-2019 at a university hospital. Deliveries of parous women with OASI were compared to deliveries without OASI. The risk factors associated with OASI were investigated.
Results: Overall, 35,397 women were included in the study with an OASI rate of 0.4% (n = 144). A higher rate of only one previous vaginal delivery was noted in the OASI group (78.5% vs. 46.4%, OR [95% CI] 4.20, 2.82-6.25, p < 0.001). The rate of vacuum-assisted deliveries was comparable between the study groups. The median birth weight was higher among the OASI group (3566 vs. 3300 g, p < 0.001), as was the rate of macrosomic neonates (19.4% vs. 5.5%, OR [95% CI] 4.15, 2.74-6.29, p < 0.001). On multivariate logistic regression analysis, only two factors were independently positively associated with the occurrence of OASI: a history of only one previous vaginal delivery (adjusted OR [95% CI] 4.34, 2.90-6.49, p = 0.001), and neonatal birth-weight (for each 500 g increment) (adjusted OR [95% CI] 2.51, 1.84-3.44, p < 0.001).
Conclusions: Among parous women, the only factors found to be independently positively associated with OASI were the order of parity and neonatal birth-weight. Vacuum-assisted delivery was not associated with an increased risk of OASI among parous women.
Keywords: Birthweight; Episiotomy; Obstetric anal sphincter injury; Parity; Parous; Vacuum-assisted delivery.