(881) Parity-specific predictors of obstetric anal sphincter injury: real-time insights for clinical decision-making

To identify maternal and intrapartum predictors of obstetric anal sphincter injury (OASIS) using clinically available data at term, stratified by parity, and to challenge the predictive value of traditional markers such as estimated fetal weight (EFW) and second-stage duration.

Study Design:

We conducted a retrospective cohort study of 22,220 singleton, term vaginal deliveries (2018–2024) at a single tertiary center. Exclusion criteria included multiple gestations and caesarean delivery before full dilation. Data included maternal demographics, pregnancy characteristics, EFW (assessed ≤7 days before delivery), and intrapartum factors. Logistic regression was used to identify independent risk factors, stratified by parity. Birthweight was excluded from models to ensure real-time clinical applicability.

Results:

OASIS occurred in 0.46% of births (102/22,220), with higher rates among nulliparas (0.92%) vs. multiparas (0.23%). In multivariable analysis of the full cohort, nulliparity (aOR 3.00, 95% CI: 1.67–5.39), vacuum-assisted delivery (aOR 2.49, 95% CI: 1.43–4.32), EFW >3500g (aOR 1.67, 95% CI: 1.01–2.76), and meconium-stained fluid (aOR 1.75, 95% CI: 1.00–3.05) were associated with increased risk. In parity-stratified models, only vacuum extraction predicted OASIS among nulliparas (aOR 2.31, 95% CI: 1.25–4.28), while prior cesarean predicted OASIS among multiparas (aOR 3.62, 95% CI: 1.31–10.00). Neither EFW nor second-stage duration remained significant in stratified models.

Conclusion:

This is one of the largest single-center OASIS cohorts to date. The study uniquely models risk using only variables available before or during labor, enhancing clinical relevance. Findings highlight that OASIS is largely unpredictable; EFW and second-stage duration have limited utility, while first vaginal birth—whether in nulliparas or VBACs—and vacuum delivery are key risks. These results support universal perineal protection strategies rather than selective risk-based approaches.