Mediation Analysis of Maternal Smoking, Gestational Age, and Birth Weight on the Texas-Mexico Border

Published data on the indirect effect of maternal smoking on birth weight as mediated by gestational age in Hispanic populations are lacking. Our goal was to conduct such a mediation analysis using data from El Paso County, Texas.

El Paso County is located on the US-Mexico border. A simple mediation analysis was conducted using year 2010 El Paso County birth certificate data. The SAS macro PROCESS 3.5.3 was used to estimate the direct and indirect effects of active maternal smoking (by trimester) on birth weight (in grams) in the setting of linear regression. The single mediator was gestational age in weeks. A direct or indirect effect was deemed to be present if the 95% confidence limits (CLs) excluded 0. Analyses were adjusted for multiple variables, including maternal prepregnancy body mass index. The indirect effect was reported along with a 95% bootstrap CL.

A total of 16,654 singleton births were included in the cohort. The majority of the mothers were White Hispanic (87.2%). The mean (standard deviation) birth weight was 3198.6 g (517.2). A direct effect of maternal smoking during each trimester on birth weight was detected. An indirect effect of maternal smoking on birth weight was not detected in any of the trimesters. In adjusted analyses for the third trimester, the indirect effect for every 1-U increase in the mean number of cigarettes smoked per day was -4.18 (95% bootstrap CL -10.64 to 1.99).

In our large, predominantly Hispanic cohort, it appears that gestational age is not a mediator of the effect of maternal smoking on birth weight. Future studies in our population should explore other possible mediators of this association.