Objective  To analyze the associations of early-pregnancy body mass index (BMI), body fat percentage, and comprehensive indexes with the risk of macrosomia under the updated international obesity definition.

Methods The study was part of the Peking University-Weifang Birth Cohort. Between March 2024 and February 2025, 650 pregnant women who had registered, undergone physical examinations during early pregnancy (6 weeks to 13 weeks and 6 days of gestation), and subsequently delivered before August 1, 2025 at Weifang Maternal and Child Health Hospital were recruited. Researchers collected basic demographic information and calculated BMI based on on-site measurements of body height and body weight during early pregnancy. Body fat percentage was assessed via bioelectrical impedance analysis (BIA) and delivery data, including gestational age, fetal sex, and birth weight, were obtained during childbirth. The obesity of mothers in early pregnancy was determined by three methods: BMI, body fat percentage, and new international definition of obesity, and the association between obesity in early pregnancy and macrosomia was analyzed by multivariate logistic regression and trend test.

Results Among the 650 participants, 26.31% were overweight and 14.00% were obese according to BMI criteria, and 66.46% had excessive body fat. The incidence of macrosomia was 8.62%. Compared with normal BMI, overweight did not have a significant association with macrosomia (P = 0.079), whereas obese pregnant women had a 4.155-fold increased risk of delivering macrosomic infants (OR = 5.155, 95%CI: 2.451–10.843). Compared with women with normal body fat percentage, those with excessive body fat had a 1.546-fold increased risk of delivering macrosomic infants (OR = 2.546, 95%CI: 1.243–5.215). Compared with the pregnant women with both normal BMI and normal body fat, those with BMI indicating overweight combined with excessive body fat had a 1.318-fold increased risk (OR = 2.318, 95%CI: 1.006–5.338), and those with BMI indicating obesity combined with excessive body fat had a 5.437-fold increased risk (OR = 6.437, 95%CI: 2.670–15.516). An increasing trend in the risk of macrosomia with increases in early-pregnancy BMI and body fat percentage was identified (P < 0.05).

Conclusions According to the new international definition of obesity that combines BMI and body fat percentage, our analysis shows a strong association between obesity in early pregnancy and macrosomia. This finding supports early prevention strategies targeting pregnant women with BMI indicating normal weight or overweight and excessive body fat.