Impact of gestational weight gain on perineal injury and obstetric anal sphincter injury among underweight primiparous women.

OBJECTIVE

To evaluate the association between gestational weight gain (GWG) and perineal trauma in primiparous women with a pre-pregnancy body mass index (BMI) <18.5.

METHODS

This retrospective cohort study included primiparous women with singleton, vertex deliveries at a tertiary medical center (2012-2023). GWG was categorized per Institute of Medicine guidelines: below (<12.5 kg, Group A), within (12.5-18 kg, Group B), and above (>18 kg, Group C). Exclusions included preterm birth, age <18 or >45, multiple gestations, stillbirths, pregnancy terminations, pregestational and gestational diabetes. The primary outcome was the rate of overall perineal injury, including all degrees of spontaneous perineal lacerations, labial tears, and episiotomy, and the secondary outcome was Obstetric anal sphincter injury (OASI).

RESULTS

Among 4034 eligible women, 49.0% were in Group A, 39.0% in Group B, and 12.0% in Group C. No significant differences were observed between GWG groups in rates of lacerations, episiotomy, or perineal repair. OASI rates were similar across all groups (1.0%-1.1%). In multivariate analysis, GWG below (odds ratio [OR] = 1.177, 95% confidence interval [CI]: 0.974-1.423) or above (OR = 1.057, 95% CI: 0.784-1.424) the recommended range was not associated with increased risk of perineal trauma. However, epidural use (OR = 1.519, P < 0.001), vacuum-assisted delivery (OR = 3.876, P < 0.001), and higher birth weight (OR = 2.124 per kg, P < 0.001) were significant risk factors. Higher BMI within the underweight range (OR = 0.855, P = 0.012) and smoking (OR = 0.569, P = 0.001) were protective.

CONCLUSION

Among underweight primiparous women, GWG does not significantly impact the risk of perineal trauma. Epidural use, vacuum-assisted delivery, and higher birth weight increase risk, while higher BMI within the underweight range and smoking are associated with reduced risk.