Nonlinear association between birth weight and overweight/obesity in children aged 3-6 years: a retrospective analysis of 13 cities in China.

Objective: The aim of this study was to explore the impact of birth weight on overweight and obesity in children aged 3-6 years across 13 cities in China, as well as the potential interactions. Methods: This study was designed as a retrospective analysis adhering to the STROBE guidelines. Data were derived from three waves of the Physical Fitness Survey in Jiangsu targeting children aged 3-6 years, which employed a multi-stage, stratified, cluster probability sampling design to obtain a representative sample from thirteen cities. Overweight and obesity were defined according to the Chinese National Health Industry Standard WS/T 423-2022 (issued in 2022) based on BMI standard deviation scores. Eight covariates, including age, sex, survey year, geographic region, urban-rural status, gestational age, childbearing age, and mother's education, were included. Multivariate logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for the risks of overweight, obesity and overweight/obesity associated with low birth weight and macrosomia after adjusting for covariates. Subgroup analysis results were visualized using forest plots; restricted cubic spline (RCS) analysis was performed to examine nonlinear and dose-response relationships; and sensitivity analysis included mixed-effects logistic regression models to adjust for clustering effects, changing birth weight classification cut-points, and excluding gestational age (a key potential covariate). Results: A total of 23,362 participants were included. The prevalence of overweight, obesity, and overweight/obesity was 19.2, 9, and 28.2%, respectively. (1) Birth weight exhibited a nonlinear association with weight and BMI at 3-6 years, with a gentle slope in the early segment and a steeper slope in the later segment (P for nonlinearity < 0.001). (2) After covariate adjustment, low birth weight was not associated with overweight risk ( Conclusion: Birth weight shows nonlinear associations with weight, BMI, and risks of obesity/overweight/obesity in children aged 3-6 years, characterized by a gentle slope in the lower birth weight range and a steeper slope in the higher range. Low birth weight has limited impact on the development of overweight and obesity, but gestational age shows significant heterogeneity; whereas macrosomia confers significantly higher risks of overweight, obesity, and overweight/obesity compared with normal birth weight, with this effect diminishing with increasing age.