[Age-stratified association between preconception body mass index and risk of macrosomia at delivery]

Objective: To investigate the impact of preconception body mass index (BMI) on neonatal birth weight and the risk of macrosomia in pregnant women across various age groups. Methods: A cohort study was conducted, selecting pregnant women who underwent their initial prenatal assessment at Beijing Obstetrics and Gynecology Hospital from September 1st, 2018 to March 31st, 2020. Relevant data were collected from the hospital's electronic medical record system. Logistic regression nested cubic spline was used to analyze the nonlinear association between preconception BMI and neonatal birth weight. Binary logistic regression was also employed to assess the association between preconception BMI and macrosomia risk. Results: (1) A total of 13 015 pregnant women were examined, revealing an incidence of macrosomia of 6.33% (824/13 015). The preconception BMI of pregnant women in the macrosomia group was significantly higher than that in the non-macrosomia group [(23.1±3.4) vs (21.6±3.1) kg/m²], and the age was significantly higher than that in the non-macrosomia group [(32.1±3.6) vs (31.7±3.7) years], the differences were statistically significant (all P<0.05). (2) Preconception BMI was positively correlated with neonatal birth weight. Pregnant women with preconception BMI of 15.0 kg/m², 20.0 kg/m², and 25.0 kg/m² had decreased birth weight of 121 g (95%CI: 35-183 g) and increased birth weights of 78 g (95%CI: 54-102 g) and 182 g (95%CI: 151-213 g), respectively, compared to those with a preconception BMI of 18.0 kg/m². (3) For each 1.0 kg/m² increase in preconception BMI, the risk of macrosomia increased by 14% (OR=1.14, 95%CI: 1.11-1.16; P<0.001). When stratified by age, it was observed that elevated preconception BMI significantly increased the incidence of macrosomia in women aged 27-38 years. Among them, the risk of delivering macrosomia among 37 years old pregnant women was most affected by preconception BMI (OR=1.33, 95%CI: 1.17-1.51; P<0.001). (4) The stability and sensitivity analysis results showed that the preconception BMI of pregnant women with a preconception BMI of 18.0-<25.0 kg/m² had a significant impact on the risk of macrosomia (OR=1.23, 95%CI: 1.17-1.29; P<0.001), while the preconception BMI of other preconception BMI stratification pregnant women had no significant impact on the risk of macrosomia (all P>0.05). Hypertension disorders in pregnancy, gestational diabetes mellitus and abnormal blood lipid during pregnancy were not the mediators associated with preconception BMI and macrosomia. After excluding three factors respectively, the impact of preconception BMI on the risk of macrosomia was the same as before (OR=1.14, 95%CI: 1.11-1.16; P<0.001). Conclusions: Preconception BMI is linked to neonatal birth weight and the risk of macrosomia, which is influenced by the pregnant woman's age. Both factors should be considered when evaluating the risk of macrosomia in clinical practice.