Joint Associations of Maternal Smoking and Pre-Pregnancy BMI with Term Low Birthweight and Small-for-Gestational-Age Births: A Cross-Sectional Analysis of 2024 United States Birth Data

Low birthweight (LBW; <2500 g) remains a persistent public health concern in the United States, with important implications for infant survival and long-term health. Maternal smoking and pre-pregnancy body mass index (BMI) are key determinants of fetal growth, yet their joint effects in term infants are not well characterized. Our analysis of the 2024 U.S. National Center for Health Statistics (NCHS) birth data restricted to 2,973,679 singleton term births (37–41 weeks) reveals that 2.8% of infants were born LBW and 8.1% small-for-gestational-age (SGA). Smoking during pregnancy (2.2%) was associated with higher odds of LBW (aOR 2.19; 95% CI: 1.92, 2.51), SGA (aOR 1.90; 95% CI: 1.85, 1.96), and combined SGA/LBW (aOR 2.75; 95% CI: 2.66, 2.84) than nonsmoking. Underweight BMI was associated with higher odds of SGA/LBW (aOR 1.95; 95% CI: 1.89, 2.01) and overweight/obesity with lower odds of SGA (aOR 0.69; 95% CI: 0.68, 0.70) than normal BMI. Joint associations of BMI and smoking on term SGA/LBW were strongest (aOR 5.14; 95% CI: 4.62, 5.73) among underweight smokers. The additive scale interaction was positive for underweight smokers and negative for overweight/obese smokers. Targeted interventions addressing smoking cessation and optimal maternal weight may reduce risks.