Abstract
BACKGROUND: Maternal weight status, before or during pregnancy, is a significant determinant of fetus development, birth weight, and the short-term and long-term health outcomes of the offspring. OBJECTIVE: This study aimed to evaluate the effect modification of pre-pregnancy body mass index (BMI) on the associations of gestational weight gain (GWG) and birth weight, as per the latest guidelines from the Chinese Nutrition Society. METHODS: This is a retrospective cohort study performed in a tertiary hospital with the largest deliveries in Shanghai, China. This study included all women who had singleton live births from 2021 to 2022 (n = 50,391). Data on pre-pregnancy weight, GWG, and birth weight were extracted from the medical register system. Logistic regression models were used to estimate the associations of pre-pregnancy BMI and GWG with the risks of being small for gestational age (SGA) and large for gestational age (LGA). The potential for effect modification by BMI on the associations of GWG and birth weight was assessed using both additive and multiplicative scales. RESULTS: Pre-pregnancy BMI and GWG were consistently associated with birth weight. We observed a positive effect modification by underweight on the relationships between insufficient GWG and SGA both in multiplicative (adjusted odds ratio (OR), 2.49, 95 % confidence interval (CI): 2.06-2.99), and additive (relative excess risk due to interaction (RERI), 3.04, 95 % CI: 1.70-4.37) scales. Similarly, obesity was found to modify the effect of excessive GWG on the risk of LGA (adjusted OR, 3.82, 95 % CI, 3.14-4.63; RERI, 14.67, 95 % CI: 7.92-21.41). CONCLUSION: Our findings indicate that increased GWG is associated with a higher risk of abnormal birth weight in singleton pregnancies. Additionally, there is evidence of an additive interaction between pre-pregnancy BMI and GWG on the risk of small for gestational age or large for gestational age.