Saturation effects of pre-pregnancy BMI on infant birth weight in Jiangxi Province, China: a retrospective study

There were limited analyses on relationship between body mass index (BMI) pre-pregnancy, as well as birth weight. Research aimed to examine the relation between these two, as well as low birth weight (LBW) risk in Jiangxi Province, China. A total of 1193 pregnant subjects from Jiangxi Province, China were enrolled in the final analysis. Standardized questionnaires were administered to the women during childbirth, newborns' medical information was obtained from hospital records. Pre-pregnancy BMI was categorized into underweight, normal, overweight, and obese groups. Multivariate linear regression models were employed to assess connection between pre-pregnancy BMI, birth weight. We utilized generalized additive model and fitted smoothing curve (penalized spline method) to examine relationship between pre-pregnancy BMI and birth weight, as well as LBW risk. The incidence of LBW was 12.1%, with average pre-pregnancy BMI of 20.9 ± 2.5 kg/m². The smoothing curve revealed an L-shaped association between pre-pregnancy BMI, birth weight and LBW risk. The curve indicated that as pre-pregnancy BMI increased, LBW risk initially decreased and then plateaued, while birth weight initially escalated and then plateaued. The inflection point for pre-pregnancy BMI was identified as 22 kg/m². On the left side of inflection point, β (95% CI) for birth weight was 0.04 (0.02, 0.07), and ORs (95% CIs) for LBW risk were 0.78 (0.69, 0.89), on the right side the corresponding values were - 0.00 (-0.03, 0.03) and 1.02 (0.88, 1.19), respectively. All the outcomes presented to be similar in various subgroups. Within a specific range (BMI<22 kg/m²), the correlation between pre-pregnancy BMI and birth weight is statistically significant. This research indicated pre-pregnancy BMI demonstrates a saturation effect on birth weight and LBW risk among Jiangxi Province population.