This study was performed to assess the risk of obstetric anal sphincter injury (OASIS) in women undergoing a vaginal birth after cesarean delivery (VBAC) compared to primiparous women and women undergoing their second vaginal delivery.METHODS:
This retrospective cohort study used data from a single institution from 2011 through 2014. Pearson's chi-square tests and two-sample t tests were used to analyze the incidence of OASIS. Adjusted multivariable logistic regression was used to adjust for differences between groups. Simple logistic regression was used to summarize the relationship between predictor variables and OASIS.RESULTS:
The incidence of OASIS was 2.6% (n=93) in 3,508 term births. The episiotomy rate was 1.0%. There was a trend toward increased risk of OASIS in women undergoing a VBAC (4.8%) compared to first vaginal delivery (3.6%), (OR 1.18, P=.677). There was an increased risk of OASIS in women undergoing a VBAC (4.8%) compared to second vaginal delivery (0.8%), (OR 3.58, P=.003). Forceps assisted delivery (OR 11.8, P<.001), vacuum assisted delivery (OR 3.36, P=.006), episiotomy (OR 3.34, P=.029) and increased birth weight (OR 1.57, P=.001) were associated with increased risk for OASIS.CONCLUSION:
There was trend toward increased risk of obstetric anal sphincter injury in women undergoing a VBAC compared to primiparous women. Prior studies have shown an increased risk of OASIS in women undergoing VBAC, however these studies had high episiotomy rates (27–65%). The episiotomy rate in this study was low (1.0%) and therefore, is more generalizable to current practice.