Hypothesis / aims of study
Gestational weight gain (GWG) is a modifiable obstetric factor with well-established implications for maternal and neonatal outcomes. The relationship between GWG and perineal injury, however, remains poorly characterized. The aim of this study was to evaluate whether GWG (above or below ACOG-recommended ranges) is independently associated with an increased risk of perineal or anal sphincter injury among nulliparous women with normal pre-pregnancy BMI.
Study design, materials and methods
A retrospective cohort study was conducted at a single, tertiary, university-affiliated medical center, including all nulliparous women with a normal pre-pregnancy BMI (18.5–24.9 kg/m²) who delivered a singleton live infant vaginally at term (≥37 weeks) between January 2011 and January 2024. Women with pregestational or gestational diabetes mellitus were excluded. The cohort was stratified into three groups according to ACOG GWG guidelines (recommended range: 11.5–16 kg): below recommended, within recommended, and above recommended. The primary outcome was overall perineal injury, including any perineal or labial tears or episiotomy. The secondary outcome was the proportion of obstetric anal sphincter injury (OASI). Multivariable logistic regression was performed to assess the independent association between GWG category and the risk of perineal injury and OASI, adjusting for confounders.
Results
A total of 26,828 women met inclusion criteria: 9,019 (33.6%) in the below-recommended GWG group, 11,527 (43.0%) in the appropriate group, and 6,282 (23.4%) in the above-recommended group. The mean GWG was 8.7±2.2 kg, 13.8±1.4 kg, and 20.0±3.2 kg, respectively. Women with GWG above recommendations had a significantly higher rate of labor induction, a longer second stage of labor, and a higher birthweight (all p<0.001). The rate of any perineal injury was modestly but significantly higher in the above-GWG group compared to the appropriate and below groups (86.4% vs. 84.9% vs. 84.2%, p<0.001). OASI rates were comparable across all three groups (0.7%, 0.7%, and 0.9%, respectively; p=0.506). On multivariable logistic regression, GWG category was not independently associated with overall perineal injury (above-recommended GWG: OR 1.021, p=0.665; below-recommended GWG: OR 1.054, p=0.197). Independent predictors of perineal injury included vacuum extraction (OR 3.288), birthweight (OR 1.957), epidural analgesia (OR 1.489), occiput-posterior position (OR 1.450), and induction of labor (OR 1.285).
Similarly, multivariable logistic regression analysis for OASI confirmed that GWG category was not an independent predictor of obstetric anal sphincter injury (above-recommended GWG: p=NS; below-recommended GWG: p=NS). Independent risk factors for OASI were vacuum-assisted delivery, occiput-posterior fetal position, and higher birthweight.
Interpretation of results
Although women with excessive GWG had a marginally higher crude rate of perineal injury, this association was entirely explained by confounding variables, most notably higher birthweight, longer second stage of labor, and higher rates of operative delivery. After adjustment, GWG was not an independent predictor of perineal trauma or OASI. This suggests that the perineal risk attributed to excessive weight gain may reflect mediated pathways rather than a direct effect of GWG on perineal tissue integrity.
Concluding message
In nulliparous women with normal pre-pregnancy BMI, gestational weight gain, whether below, within, or above ACOG recommendations, does not independently influence the risk of perineal tears or OASI. Clinically relevant predictors of perineal injury include vacuum extraction, birthweight, epidural analgesia, and occiput-posterior presentation. These findings suggest that counseling regarding perineal injury risk should focus on intrapartum factors rather than GWG in this population.