Pre-Pregnancy BMI and Gestational Weight Gain Have Independent Associations with Birth Weight: A Prospective Cohort Study in Mongolia

Background: Neonatal birth weight is a key determinant of short- and long-term health outcomes. Pre-pregnancy body mass index (BMI) and gestational weight gain (GWG) are important predictors of birth weight; however, their independent associations remain unclear, particularly in settings where national GWG guidelines are lacking. Methods: A prospective cohort study was conducted among 340 pregnant women in Ulaanbaatar, Mongolia in 2022. Data were collected using standardized questionnaires and anthropometric measurements. Pre-pregnancy BMI and GWG were classified according to World Health Organization criteria and the 2009 Institute of Medicine guidelines. Multivariable linear regression models were used to examine independent associations with neonatal birth weight. Results: The mean maternal age, pre-pregnancy BMI, and GWG were 30.3 ± 6.0 years, 23.5 ± 4.4 kg/m2, and 14.3 ± 6.2 kg, respectively. Both BMI and GWG were independently associated with neonatal birth weight (p < 0.001), with no significant interaction observed (p = 0.147). Birth weight increased by 29.7 g (95% CI: 18.6–40.8) per 1 kg/m2 increase in BMI and by 31.7 g (95% CI: 24.0–39.4) per 1 kg increase in GWG. Compared with normal BMI, overweight/obesity was associated with higher birth weight, while excessive GWG increased and inadequate GWG decreased birth weight (all p < 0.05). Conclusions: Pre-pregnancy BMI and GWG were independently associated with neonatal birth weight, with GWG showing a slightly stronger association. These findings highlight the importance of optimizing maternal weight before and during pregnancy. Countries undergoing rapid nutritional transitions may benefit from developing context-specific GWG guidelines, with Mongolia representing a relevant example.