Predictors and Unexplained Variability of Obstetric Anal Sphincter Injuries: A Population-Based Cross-Sectional Analysis.

Introduction and hypothesis: The objective was to determine the incidence of obstetric anal sphincter injuries (OASIs) and assess the predictive power of clinical risk factors for OASIs, both individually and collectively, using a US population-based birth registry.

Methods: We conducted a cross-sectional study using birth data from the National Vital Statistics System from 2016 to 2021. All vaginal births were included, excluding those with unspecified delivery route or perineal laceration status. Logistic regression models assessed the relationship between potential clinical predictors, determined a priori based on existing literature and inclusion in the registry data, and OASIs. The pseudo-R² (pR²), which indicates the proportion of variability in OASIs explained by the model out of 100%, was used to compare the predictive strength of different clinical factors.

Results: A total of 15,413,957 vaginal births were analyzed. The overall incidence of OASIs was 1.1%. The factors that exhibited the highest predictive capability for OASIs were vaginal parity (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. The final multivariable logistic regression model had a pR² of 11.8%.

Conclusion: Vaginal parity and operative birth are among the most predictive clinical risk factors for OASIs, each accounting for approximately 5% of the variation in OASI outcomes. However, a significant portion of OASI occurrences remains unexplained by clinical risk factors alone. Future research focusing on the mechanisms of OASIs will be crucial for identifying high-risk patients and developing targeted interventions to reduce the risk of OASIs.