Pre-pregnancy body mass index (BMI) is a well-established determinant of maternal and perineal outcomes. While the adverse effects of obesity on labor are widely recognized, the relationship between pre-pregnancy underweight (BMI <18.5 kg/m²) and perineal trauma, particularly obstetric anal sphincter injury (OASI), remains poorly characterized in nulliparous women. The aim of this study was to evaluate whether pre-pregnancy underweight is independently associated with overall perineal injury and OASI in nulliparous women.

A retrospective cohort study was conducted at a single tertiary center, including all nulliparous women aged 18–45 years with a singleton pregnancy and normal (18.5–24.99 kg/m²) or underweight (BMI <18.5 kg/m²) pre-pregnancy BMI who delivered vaginally at term (≥37 weeks) between January 2012 and January 2024. Women with any type of diabetes mellitus were excluded. The cohort was divided into two groups: underweight and normal BMI. The primary outcome was overall non-severe perineal injury, defined as any combination of first- and second-degree perineal lacerations, labial tears, and episiotomy. The secondary outcome was severe perineal injury, defined as OASI (third- and fourth-degree tears). Data were extracted from electronic medical records and included maternal demographic characteristics, obstetric variables, and neonatal outcomes. Multivariable logistic regression was performed to assess the independent association between pre-pregnancy BMI category and both primary and secondary outcomes, adjusting for maternal age, prolonged second stage, occiput-posterior position, epidural analgesia, induction of labor, vacuum-assisted delivery, and birthweight.

A total of 32,112 women were included: 4,160 (12.9%) in the underweight group and 27,952 (87.1%) in the normal BMI group. Underweight women had significantly lower birthweight, lower rates of induction of labor, and lower rates of occiput-posterior presentation compared to normal-BMI women (all p<0.05). The rate of overall non-severe perineal injury was significantly lower in the underweight group compared to the normal BMI group (81.5% vs. 84.3%, p<0.001). Conversely, OASI was significantly more frequent in underweight women (1.1% vs. 0.8%, p=0.033). On multivariable logistic regression, pre-pregnancy underweight was independently associated with a reduced risk of overall perineal injury (OR 0.907, 95% CI 0.827–0.994, p=0.037) and with a significantly increased risk of OASI (OR 1.497, 95% CI 1.065–2.103, p=0.02). Additional independent risk factors for OASI included vacuum-assisted delivery (OR 5.235), birthweight (OR 6.267 per kg), and occiput-posterior position (OR 3.688).

Pre-pregnancy underweight appears to exert a dual and paradoxical effect on perineal outcomes: it is associated with a modest reduction in overall perineal injury, yet simultaneously confers a significantly elevated risk of OASI. This dissociation suggests that underweight may compromise the structural resilience of the anal sphincter complex independently of perineal trauma mechanisms.

In nulliparous women, pre-pregnancy underweight is independently associated with a lower overall rate of perineal injury but a significantly higher risk of OASI. These findings highlight the importance of pre-pregnancy nutritional counseling and suggest that intrapartum care for underweight women should include heightened vigilance for anal sphincter injury.

Continence 19S (2026) 102529
DOI: 10.1016/j.cont.2026.102529