The impact of pre‐pregnancy underweight on the risk of perineal and anal sphincter injury in nulliparous women

Introduction Perineal injury is a common complication of vaginal delivery with maternal consequences. Although the effects of obesity are established, the impact of underweight on perineal and obstetric anal sphincter injury (OASI), remains unclear. Objective Our objective was to determine the association between pre‐pregnancy underweight and the risk of non‐severe perineal injury (NSPI) and OASI in nulliparous women. Methods A retrospective cohort study was conducted at a single, tertiary medical center including nulliparous women aged 18–45 years with singleton term vaginal deliveries (≥37 weeks) and pre‐pregnancy body mass index (BMI) <25 kg/m 2 between 2012 and 2024. Women with diabetes were excluded. The cohort was divided into underweight (BMI <18.5 kg/m 2 ) and normal BMI groups. The primary outcome was NSPI, and the secondary outcome was OASI. Multivariable logistic regression was performed to assess independent associations. Results Overall, 32 112 women were included, 12.9% of whom were underweight. Underweight women had lower birthweights and lower rates of induction and occiput‐posterior presentation (all P < 0.05). NSPI occurred slightly less frequently among underweight women than among women with normal BMI (81.5% vs 84.3%, P < 0.001), whereas OASI was more common (1.1% vs 0.8%, P = 0.033). In the fully adjusted model, underweight BMI was associated with slightly lower odds of NSPI (adjusted odds ratio [aOR] 0.905; 95% confidence interval [CI] 0.825–0.994) and higher odds of OASI (aOR 1.641; 95% CI 1.179–2.286). Additional factors associated with OASI in the fully adjusted model included higher birthweight per 100 g increment (aOR 1.132; 95% CI 1.096–1.169), occiput‐posterior presentation (aOR 2.815; 95% CI 1.806–4.386), prolonged second stage (aOR 1.590; 95% CI 1.158–2.182), and vacuum‐assisted delivery (aOR 2.304; 95% CI 1.666–3.187). Conclusion Pre‐pregnancy underweight was associated with slightly lower odds of NSPI but higher odds of OASI.