Risk Factors for Obstetric Anal Sphincter Injury in a Diverse Population [1C

Abstract

INTRODUCTION: We evaluated risk factors for obstetric anal sphincter injuries (OASIS) and determined the effect of duration of second stage of labor (DSS) on OASIS risk. METHODS: We included term, singleton, cephalic vaginal deliveries within Kaiser Permanente Northern California over two years. Bivariate analysis identified OASIS risk factors. Multiple logistic regression helped eliminate confounding. Classification and Regression Tree (CART) recursive partitioning created decision nodes based on risk factors. DSS was grouped into 15-minute intervals to examine effect magnitude. RESULTS: Of 22,741 deliveries included, 93.8% were spontaneous and 6.2% were vacuum-assisted. The OASIS rate was 4.9%. Multiple logistic regression identified Asian race (OR 2.4, CI 2.1-2.8), vacuum delivery (OR 4.6, CI 3.9-5.4), DSS greater than 75 minutes (OR 2.4, CI 2.1-2.8), VBAC (OR 2.8, CI 2.1-3.8), and nulliparity (OR 1.7, CI 1.4-2.1) as independent risk factors for OASIS. DSS ranged from 0-680 minutes. Median length was 95 minutes for nulliparous and 27 minutes for multiparous women. The first CART decision node was DSS greater than 75 minutes. Each additional 15 minutes of pushing resulted in 1.07 (p < 0.001) increase in the odds of OASIS in nulliparous women and 1.14 (p < 0.001) in multiparous women. CART analysis identified Asian women delivered via vacuum after a greater than 75 minute second stage as the subpopulation with highest risk of OASIS at 43%. CONCLUSION: Risk factors for highest rates of OASIS were identified. Each incremental increase in DSS confers an increase in OASIS risk. Changing management based on our findings could improve outcomes for laboring women.