Objective: To analyze the effects of maternal gestational weight gain and pre pregnancy body mass index (BMI) on the weight of preschool children,so as to provide scientific basis for prevention and treatment of overweight and obesity in children. Methods: Based on Jiangmen maternal and child health information platform, annual physical examination data of 3-6 years old preschool children from all nurseries and kindergartens in Jiangmen were collected from January to December 2024. A unique identification was made according to the mother s ID number and delivery date, and retrospective data collection was conducted on the platform to obtain pre pregnancy and pregnancy related information for 46 481 mothers. The Chi-square test,two way ordered variable analysis and Logistic regression analysis were used to compare the effects of maternal pre pregnancy BMI and gestational weight gain on overweight and obesity among preschool children. Results: A total of 5 168 (11.12%) children were overweight and obese, and the proportion of overweight and obesity in the 6 year old group was the highest (12.86%). There were significant differences in the detection rates of overweight and obesity between boys and girls ( χ 2=155.38), and there were also significant gender differences in the age groups of 4, 5 and 6 years ( χ 2=17.08, 96.97, 66.27)(all P <0.01). Through trend χ 2 test, the overall detection rates of overweight and obesity, as well as those for boys, increased with age ( χ 2 trend =49.36,60.54, both P <0.01). The BMI group of preschool children was correlated with the BMI group of their mothers before pregnancy and the weight gain group during pregnancy （χ 2= 1 250.64, 157.01, both P <0.01) and the proportion of children with higher BMI levels showed an upward trend with the improvement of their mothers pre-pregnancy BMI levels or gestational weight gain levels ( Gamma =0.13, 0.10, both P <0.01). Multiple Logistic regression analysis showed that pre pregnancy BMI groups as overweight ( OR =1.590, 1.922), obesity ( OR =2.100, 2.921 ), and male gender of the children ( OR =1.213, 1.763)，and newborns excessive birth weight( OR =1.001，1.001) increased the risks of overweight and obesity in preschool children; maternal gestational weight gain insufficiency ( OR =1.374) and advanced maternal age at the first prenatal visit ( OR =1.012) increased the risks of obesity in preschool children; maternal gestational weight gain deficiency or excess ( OR =1.324,1.118) increased the risk of overweight in preschool children (all P <0.01). Conclusions Maternal pre-pregnancy overweight and obesity and insufficient or excessive gestational weight gain increase the risk of overweight and obesity in preschool children. It is necessary to strengthen weight management before and during pregnancy to reduce the occurrence of childhood overweight and obesity.