Association Between Gestational Weight Gain and Obstetric Anal Sphincter Injury

OBJECTIVE: To investigate the association between gestational weight gain across all prepregnancy body mass index (BMI) categories and the risk of obstetric anal sphincter injury during vaginal birth. METHODS: Our retrospective population-based cohort study included individuals with singleton, term, vaginal births in the United States (2021-2023) and used data from the National Vital Statistics System's natality files. We examined the association between gestational weight gain (below, within, or above Institute of Medicine guidelines) and obstetric anal sphincter injury across BMI categories using logistic regression models. We assessed nonlinear associations between gestational weight gain and obstetric anal sphincter injury using restricted cubic spline regression and conducted a mediation analysis to examine the influence of fetal birth weight. RESULTS: Obstetric anal sphincter injury occurred in 1.2% of births, with rates of 1.3% among those who gained above the recommendations, 1.2% among those who gained within the recommendations, and 1.0% among those who gained below the recommendations. Gestational weight gain above and within guideline recommendations were associated with increased odds of obstetric anal sphincter injury among individuals with average-weight (adjusted odds ratio [aOR] 1.08; 95% CI, 1.05-1.10), overweight (aOR 1.13; 95% CI, 1.09-1.17), class I obesity (aOR 1.12; 95% CI, 1.06-1.18), and class II obesity (aOR 1.15; 95% CI, 1.06-1.26) BMIs. Gestational weight gain below and within recommendations showed a protective effect against obstetric anal sphincter injury only in individuals with average-weight BMIs (aOR 0.91; 95% CI, 0.88-0.93). Mediation analyses showed that gestational weight gain indirectly affected obstetric anal sphincter injury risk through birth weight across all BMI groups; direct effects were observed only in individuals with underweight, average-weight, and overweight BMIs. CONCLUSION: Gestational weight gain above the Institute of Medicine guideline was associated with increased risk of obstetric anal sphincter injury across all BMI categories except for underweight and class III obesity; weight gain below the guideline appeared to be protective only at average weight. These associations were largely mediated through birth weight.