Obstetric Anal Sphincter Injuries in Vaginal Birth After Cesarean versus First Vaginal Delivery

Objectives: There is limited and contradictory evidence for an increased risk of obstetric anal sphincter injuries (OASIS) after vaginal birth after cesarean delivery (VBAC). Our objective was to compare the odds ratio of OASIS with primiparous vaginal deliveries versus the first VBAC. We hypothesized that VBAC would have a higher proportion of OASIS compared to primiparous vaginal delivery.

Methods: We performed a retrospective cohort study of liveborn deliveries from 2018-2022 within a large, multi-hospital academic health system. We collected information from electronic health records using the health system’s clinical data warehouse. All patients had a primiparous vaginal delivery or a first VBAC at ≥37 weeks and received prenatal care within our system. We excluded patients with any prior vaginal delivery and more than one prior cesarean delivery.

Our primary outcome was the rate of OASIS (3^rd or 4^th degree obstetric perineal laceration) compared between patients who underwent a first vaginal delivery (primiparous vaginal delivery group) versus patients who underwent a vaginal delivery following one cesarean delivery (VBAC group). Secondary outcomes included operative vaginal delivery, maternal length of stay, severe maternal morbidity (a composite outcome utilizing diagnosis and procedure codes defined by CDC criteria), and neonatal ICU admission. We performed t-tests, chi-square, and multivariable logistic regression (controlling for age, weight, race, ethnicity, birthweight, operative vaginal delivery, delivery hospital, and year) with Stata 16. Our institutional research board approved this project.

Results: Our cohort consisted of 16,475 patients with a first primiparous vaginal delivery versus 843 with a first VBAC. VBAC patients were older, more likely to self-identify as Black or Asian race, had lower maternal weight (p < 0.01), and slightly higher birthweights (p=0.04). OASIS were significantly more common in the VBAC group compared with the primiparous vaginal delivery group (7.00% vs. 5.10%, p=0.02). Vacuum-assisted vaginal deliveries were also more common in the VBAC group (7.6% vs. 5.4%, p< 0.01). There were no differences in maternal morbidity or neonatal ICU admission (Table 1).

In the multivariable logistic regression model, VBAC was no longer statistically significantly associated with OASIS (OR 1.01, CI 0.75-1.36, p=0.95, Table 2). However, operative vaginal delivery was strongly associated with OASIS (OR 6.44, CI 5.51-7.52, p< 0.01). The significant difference in crude OASIS rates between VBAC and prime vaginal deliveries did not persist on multivariable regression modeling, partly due to an increased frequency of vacuum-assisted operative deliveries in the VBAC cohort.

Conclusions: VBAC delivery is not an independent risk factor for OASIS. OASIS is more common among VBAC patients, but these patients were more likely to undergo operative vaginal delivery, specifically vacuum-assisted vaginal delivery. Therefore, patients considering a trial of labor after cesarean delivery should be counseled regarding an increased possibility of operative vaginal delivery and the associated risks, including OASIS.