Association Between Prepregnancy Body Mass Index and Adverse Pregnancy Outcomes Among Women with Inadequate Gestational Weight Gain: A Retrospective Cohort Study

Background: Inadequate gestational weight gain (GWG) is a well-established risk factor for adverse maternal and neonatal outcomes. Although pre-pregnancy body mass index (BMI) informs clinical GWG recommendations, its role as an effect modifier—particularly in women with inadequate GWG—remains poorly characterized. This study aimed to evaluate how pre-pregnancy BMI modifies the risks of maternal and neonatal complications in this high-risk population. Methods: We conducted a retrospective cohort study using data from 865 Chinese women with inadequate GWG, stratified by pre-pregnancy BMI into underweight (n = 233), normal-weight (n = 395), and overweight/obese (n = 237) groups. Multivariable logistic regression models were employed to estimate associations between BMI categories and pregnancy outcomes, adjusting for maternal age and total GWG. Results: Compared with the normal-weight group, pre-pregnancy overweight/obesity was significantly associated with higher risks of cesarean delivery (adjusted odds ratio [aOR] = 2.23; 95% CI: 1.55–3.21) and hypertensive disorders of pregnancy (aOR = 4.22; 95% CI: 2.20–8.11). Pre-pregnancy underweight was independently associated with increased risk of neonatal low birth weight (aOR = 2.78; 95% CI: 1.56–4.95). The association between overweight/obesity and macrosomia was not statistically significant after adjustment. No significant associations were observed for preterm birth or other assessed outcomes. Conclusion: Among women with inadequate GWG, pre-pregnancy BMI serves as a critical effect modifier of adverse pregnancy outcomes. Overweight/obesity predominantly elevates maternal morbidity, whereas underweight status confers substantial neonatal risk. These findings underscore the need for BMI-stratified clinical management in pregnant women with inadequate GWG to support targeted, evidence-based interventions.