Fetal Maternal Size Disproportion: Explanation for the Racial Disparity of OASIS? [7R

Abstract

INTRODUCTION: Obstetric anal sphincter injuries (OASIS) are a significant complication of vaginal deliveries. Asian race is a known independent predictor of perineal trauma. Limited data demonstrate a plausible explanation of this racial disparity. We aim to assess the relationship between the fetal maternal size disproportion as measured by fetal birth weight to prepregnancy BMI ratio (BW:BMI) and OASIS in mothers of different race groups. METHODS: Retrospective cohort study of vaginal deliveries between 2006 and 2015. Demographics and obstetrical data were abstracted and compared. The significance of BW:BMI to OASIS was evaluated. Results adjusted for confounding factors. RESULTS: 16,024 vaginal deliveries were included. 28.5% had no lacerations, 64.6% had 1st or 2nd degree lacerations, and 6.9% had OASIS. Demographic factors associated with OASIS included Asian race, nulliparity, increased age, increased birth weight, episiotomy, trial of labor after cesarean, and operative vaginal deliveries. Although BMI was not an independent risk factor, BW:BMI showed a significant relationship with OASIS. For every 10 unit increase in BW:BMI, the adjusted odds of having an OASIS increased by 16% (p= < .001). After adjusting for BW:BMI, Asian race had higher OASIS rates. Interaction between race groups and BW:BMI was only significant for 1st and 2nd degree lacerations, not OASIS. CONCLUSION: This study verified known risk factors associated with OASIS. Increasing BW:BMI is associated with OASIS. However, BW:BMI does not provide insight into OASIS racial disparities; further investigations are needed to understand why Asian race is an OASIS risk factor.