The association between BMI, episiotomy, and obstetric anal sphincter injuries (OASIS) in singleton vaginal deliveries

Objective: Prevalence of obesity in reproductive age and pregnant women has been on the rise during the past several decades. The relationship between body mass index (BMI) and obstetric anal sphincter injuries (OASIS) or episiotomy has not yet been thoroughly investigated. The objective of this study was to shed light on this issue. Methods: This retrospective cohort study was performed using electronic database of an obstetrics department at a university-affiliated tertiary medical center. All spontaneous singleton vaginal deliveries at term between January 2015 and December 2021 were included. The primary outcome was the incidence of OASIS (third- and fourth-degree perineal tears) and obesity. These were compared across three BMI categories: normal (BMI below 25 kg/m²), overweight (BMI 25.0–29.9 kg/m²), and obesity (BMI of 30 kg/m² and over). Results: Of the overall 13,932 spontaneous vaginal deliveries identified, 34.7 % had BMI in the normal range, 43.1 % were overweight, and 25.4 % were obese. Multivariate analysis demonstrated lower rates of OASIS in obese (OR 0.2, 95 %CI 0.04–0.9) and in overweight (OR 0.3, 95 %CI 0.1–0.99) women, as compared to the normal-weight cohort. In addition, lower rates of episiotomy were noted in the obese cohort (OR 0.7, 95 %CI 0.6–0.8). Conclusions: Decreased incidence of OASIS was noted in women with obesity, in conjunction with lower use of episiotomy. These findings imply that obstetrics clinics might consider a more conservative approach to episiotomy in obese patients.