Risk of anal sphincter injury in trial of labor post cesarean section

We wanted to evaluate whether secundiparas who achieved vaginal birth after cesarean (VBAC) were at an increased risk for obstetric anal sphincter injury (OASI) compared to primiparas who delivered vaginally, with a stratification by the mode of delivery-spontaneous or operative vaginal delivery.

We conducted a retrospective cohort study of primiparous women who delivered by vacuum-assisted delivery between March 2011 and June 2019. Primiparas delivering vaginally and secundiparas undergoing VBAC were compared. The cohort was further stratified into two categories: spontaneous vaginal delivery and operative vaginal delivery.

Overall, 23 822 primiparas who delivered vaginally and 1596 secundiparas who underwent VBAC were analyzed. Operative vaginal delivery was performed in 4561 deliveries. OASI rate did not differ between the VBAC and primipara groups (1.3% vs 1.8%, P = .142). A total of 20 857 women delivered by spontaneous vaginal delivery, among them 1180 (5.7%) women were secundiparas and 19 677 (94.3%) were primiparas. OASI rate was comparable between the secundiparas undergoing VBAC and primiparas delivering vaginally (17 [1.4%] vs 338 [1.7%], P = .436). A total of 4561 women delivered by operative vaginal delivery, among them 416 (9.1%) were secundiparas and 4145 (90.9%) were primiparas. The rate of operative vaginal deliveries was higher among the VBAC group compared with the primipara group (6.1% vs 17.4%, P < .001). However, women undergoing successful VBAC had lower rates of OASI compared with primiparas (3 [0.7%] vs 96 [2.3%]; odds ratio [OR] 0.30, 95% CI 0.09-0.97, P = .032). After multivariate logistic regression including all statistically significant factors, OASI was not associated with VBAC in spontaneous or operative vaginal deliveries (adjusted OR 0.85, 95% CI 0.51-1.40 and 0.39, 95% CI 0.12-1.28, respectively).

Secundiparas undergoing VBAC were not at a higher risk of OASI when compared with primiparas delivering vaginally, either in spontaneous or operative vaginal deliveries. This information might aid when counseling women contemplating a trial of labor after cesarean--to address their concerns regarding the risks and benefits of VBAC.