The relationship between maternal body mass index and tobacco use on small-for-gestational-age infants.

We sought to estimate the association between prepregnancy body mass index (BMI) and small-for-gestational-age (SGA) neonates and to determine if there is a synergistic effect of tobacco use on SGA across all BMI strata. We performed a retrospective cohort study of 65,104 patients seen for second-trimester ultrasound. BMI was categorized into underweight, normal weight, overweight, and obese. SGA was defined as birth weight <10th percentile and <5th percentile. Univariable and multivariable logistic regression analyses were used to evaluate the association between BMI and SGA. Stratified analyses and tests for effect modification were performed to evaluate for a potential synergistic effect between tobacco use and abnormal prepregnancy BMI on SGA. After controlling for potential confounders, underweight BMI was associated with an increased risk for SGA <10th percentile (adjusted odds ratio [aOR] 1.8, 95% confidence interval [CI] 1.5 to 2.1), while overweight (aOR 0.7, 95% CI 0.7 to 0.8) and obese BMIs (aOR 0.6, 95% CI 0.5 to 0.7) were associated with a decreased risk of SGA. There was no effect modification of tobacco use on the risk of SGA across all BMI categories. Although both tobacco and underweight BMI are independently associated with SGA, there was no evidence of synergism. Continued emphasis on both smoking cessation and maintenance of normal prepregnancy BMI remains paramount to decreasing the incidence of SGA.