The impact of gestational weight gain on perineal injury in primiparous women with normal pre‐pregnancy BMI

OBJECTIVE: Our objective was to determine the association between gestational weight gain (GWG) and the risk of perineal injury in nulliparous women with normal pre-pregnancy body mass index (BMI). METHODS: ) who delivered a singleton, live, term infant (≥37 weeks) vaginally, excluding pregestational or gestational diabetes. Participants were stratified by GWG per American College of Obstetricians and Gynecologists guidelines (recommended 11.5-16 kg) into below-, within-, and above-recommended GWG groups. The primary outcome was perineal injury (1st-4th degree). Multivariable logistic regression assessed the independent association between GWG and perineal trauma. RESULTS: Of 26 828 women, a total of 9019 (33.6%) had below-, 11 527 (43.0%) within-, and 6282 (23.4%) above-recommended GWG. Overall perineal injury was more common with above-recommended GWG (86.4% vs. 84.9% vs. 84.2%, P = 0.001), whereas obstetric anal sphincter injury (OASI) rates were comparable (0.9% vs. 0.7% vs. 0.7%, P = 0.506). The above-recommended group had higher rates of induction and epidural analgesia, longer second-stage duration, and higher birthweight (all P < 0.001). After adjustment, GWG category was not independently associated with overall perineal injury (above-recommended: odds ratio [OR] 1.021; 95% confidence interval [CI] 0.930-1.120, P = 0.665) or OASI (above-recommended: OR 1.065; 95% CI 0.743-1.526, P = 0.733). Independent risk factors for perineal injury included vacuum extraction (OR 3.288), epidural analgesia (OR 1.489), occiput-posterior position (OR 1.450), induction (OR 1.285), and higher birthweight (OR 1.957 per kg). CONCLUSION: Among nulliparous women with normal pre-pregnancy BMI, GWG, whether below, within, or above American College of Obstetricians and Gynecologists recommendations, did not independently affect the risk of perineal tears or OASI. Intrapartum and fetal factors remain the primary determinants of perineal injury.