(1038) Population-based incidence of obstetric anal sphincter injuries and associated risk factors

Objective: Although systematic reviews and meta-analyses have provided insight on the incidence and risk factors of obstetric anal sphincter injuries (OASIS), there are few contemporary population-based studies. Our aim was to assess the population-based incidence of OASIS and its associated risk factors.

Study Design: We performed a retrospective case-control study using birth certificate data from the National Vital Statistics System from 2016 to 2021, which encompasses all recorded births in the United States. The incidence of OASIS among vaginal births was determined. Logistic regression was used to assess the relationship between risk factors and OASIS. The pseudo-R² (pR²) was used to estimate the goodness of fit of univariate models, enabling a comparative assessment of the factors’ association with OASIS.

Results: A total of 15,413,957 were vaginal births were analyzed after excluding a negligible proportion (30,211; < 0.01%) for unknown data. The overall incidence of OASIS was 1.1%. The factors that exhibited the strongest predictability for OASIS were birth history (pR² 5.8%) and operative birth (pR² 4.8%), followed by infant weight (pR² 1.5%); all other factors had a pR² of 0.5% or lower. Compared to primiparous patients, multiparous patients with a history of only cesarean birth(s) had a higher likelihood of OASIS (aOR 1.11, 95% CI 1.08-1.15), while those with any history of vaginal birth were approximately five times less likely to have an OASIS. Compared to spontaneous births, forceps-assisted births had the highest risk of OASIS (13.0% vs. 0.9%; OR 14.51, 95% CI 14.24-14.79), and this risk remained substantially elevated in a multivariable regression model (aOR 8.76, 95% CI 8.58-8.93). Vacuum-assisted births were also associated with an increased risk of OASIS (aOR 3.73, 95% CI 3.68-3.79).

Conclusion: In this population-based study of over 15 million vaginal births, birth history and operative birth were most strongly predictive of OASIS, which occurred in 1.1% of births. Notably, use of forceps was independently associated with a 9-fold increase in the risk of OASIS, higher than previously cited.