Racial and ethnic disparities in obstetric anal sphincter injury: cross-sectional study in the USA.

Objectives: Racial disparities in obstetric anal sphincter injury (OASI) are poorly understood; their investigation by parity, obstetric history and mode of delivery may provide insight into which individuals are at the greatest risk for OASI. We aimed to quantify the association of race and ethnicity with OASI, stratified by parity, obstetric history and mode of delivery. Secondary aims were to explore variations in OASI rates among racial subgroups and by immigration status (foreign-born vs USA-born).

Methods: We conducted a cross-sectional study of 12 501 183 vaginal births in the USA from January 2016 to December 2021 using birth-certificate data obtained from the National Vital Statistics System. Cox proportional hazard regression models were fitted, with gestational age as the timescale, to quantify the association of self-reported race and ethnicity with OASI, with adjustment for several confounders. The maternal race and ethnicity groups included: American Indian or Alaska Native (AIAN), Asian, Black, Hispanic, Native Hawaiian and other Pacific Islander, White and mixed race. Models were stratified by number of previous births and the occurrence of Cesarean delivery (CD) among prior births. This resulted in three groups: primiparous (i.e. only the index birth); multiparous without a previous CD; and multiparous with at least one previous CD. Within each stratum, we further grouped individuals by mode of delivery in the index birth, as spontaneous vaginal delivery (SVD), operative vaginal delivery (OVD) with forceps and OVD with vacuum.

Results: In primiparous individuals who had a vaginal birth, the overall OASI rate was 2.2%, but it varied widely by mode of delivery (SVD, 1.7%; OVD with forceps, 14.8%; OVD with vacuum, 6.6%). Asian primiparae had higher OASI hazards compared with White primiparae, irrespective of mode of delivery (SVD: adjusted hazard ratio (aHR), 1.69 (95% CI, 1.64-1.73); OVD with forceps: aHR, 1.48 (95% CI, 1.38-1.58); OVD with vacuum: aHR, 1.51 (95% CI, 1.44-1.58)), while AIAN and Black primiparae had inconsistent associations with OASI rate depending on mode of delivery, when compared with White primiparae. In multiparous individuals without a previous CD, the rates of OASI were lower than those seen in primiparae (SVD, 0.5%; OVD with forceps, 7.5%; OVD with vacuum, 3.2%) and the association of race and ethnicity with OASI varied by mode of delivery for all race groups except Asian, in whom it was consistently associated with a 1.5-2.1-times higher hazard of OASI. Among multiparous individuals with a previous CD, overall OASI rates were similar to those seen in primiparae (SVD, 1.3%; OVD with forceps, 11.8%; OVD with vacuum, 5.1%). In this group, the only associations of race and ethnicity with OASI were higher hazards among Asian vs White individuals who had a SVD (aHR, 2.16 (95% CI, 1.97-2.36)) and an OVD with vacuum (aHR, 1.65 (95% CI, 1.39-1.96)). The rate of OASI varied widely between Asian race subgroups, with the highest rate noted among individuals with origins and/or ancestry from India (e.g. 27.2% among primiparae who had OVD with forceps) and the lowest in those from Japan (e.g. 9.3% among primiparae who had OVD with forceps). Across racial and ethnic minority groups, the OASI rate was higher among foreign-born vs USA-born residents; this trend was not observed among White individuals.

Conclusions: Racial and ethnic disparities in OASI persist regardless of parity, obstetric history and mode of delivery. Variations in the OASI rate are apparent within Asian racial subgroups and by immigration status. © 2025 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.