Background
Maternal obesity is an important comorbidity in contemporary obstetrics practice and is associated with significantly increased perinatal complications. Obstetric anal sphincter injury (OASIS) sustained during labour can lead to faecal incontinence, chronic pain and effects on quality of life. Currently, it is unclear if maternal body mass index (BMI) influences the risk of sustaining OASIS.

Aim
To investigate the impact of increased BMI on the rate of OASIS among nulliparous women.

Materials and Methods
A retrospective cohort study was conducted, which included nulliparous women with singleton, vaginal deliveries ≥37 weeks gestation at a Victorian regional centre between 2007 and 2017 (n = 3335). Logistic regression was performed to calculate unadjusted and adjusted odds ratios (aOR). Women were grouped by World Health Organization BMI categories, and the rates of OASIS were evaluated.

Results
Women with a BMI ≥ 25 were significantly less likely to develop OASIS compared to women with a BMI < 25 (aOR 0.60, 95% CI 0.41–0.89). Women with a BMI ≥ 35 had significantly decreased odds of OASIS compared to normal weight women (aOR 0.27, 0.10–0.78). While not statistically significant, the odds of OASIS decreased with each increase in BMI class. Other statistically significant risk factors were maternal age, birth weight, forceps delivery, non‐smokers (aOR 4.03, 1.46–11.1) and diabetes mellitus (aOR 9.48, 2.1–41.4). Conclusion Women with a BMI ≥ 25 were less likely to sustain OASIS compared to women with a BMI < 25. Furthermore, the odds of OASIS decreased for each increase in BMI category. These findings warrant further investigation into the mechanism of this protective effect.