Maternal Smoking and Infant Low Birth Weight: Exploring the Biological Mechanism Through the Mother’s Pre-pregnancy Weight Status

Maternal smoking has been found to adversely affect birth outcomes, such as increasing the odds of having low birth weight infants. However, the mechanisms explaining how a mother’s smoking is linked to a child’s low birth weight status are underexplored. This study merged two nationally representative datasets in the United States (US)—the National Longitudinal Survey of Youth 1979 (NLSY79) and the NLSY79 Child and Young Adult (NLSYCYA)—to examine whether maternal weight status before pregnancy serves as a biological mechanism. We applied a recently developed mediation analysis technique to a data sample of 6550 mother–child pairs, and we compared the estimated coefficients across nested probability models. We found that maternal body mass index (BMI) (in kg/m2), a widely used measure of weight status, reduces the odds of delivering a low birth weight infant, and this mechanism explains about 10.2% of the adverse impact of maternal smoking on having a low birth weight child. Moreover, when categorizing maternal pre-pregnancy BMI into four weight statuses (i.e., underweight, normal weight, overweight, and obese), we found that, in contrast to mothers with normal weight status, underweight mothers are 70% more likely to have a low birth weight child. Our findings suggest that maternal weight status plays a role in understanding how maternal smoking affects low birth weight outcome, indicating that maintaining a proper weight status for women who plan to give birth may be a possible policy to promote infant health.