Maternal Smoking by Trimester of Pregnancy and Risk of Small-for-Gestational-Age

Background: Infants born small-for-gestational-age (SGA) are at increased risk for severe disabilities and chronic health problems. Smoking and hypertension during pregnancy pose some of the most significant risk factors for fetal growth restriction. The aims of this study are to identify 1) whether the timing and dosage of maternal tobacco use modifies the risk of SGA, 2) whether there are differences in association by percentile of SGA (3^rd, 5^th and 10^th percentile), and 3) whether the effect of tobacco exposure on SGA outcome is mediated by the presence of hypertension.  

Methods: Data were obtained from the 2009 Natality public use file available through National Center for Health Statistics. Multivariable log-linear regression models were performed to determine the association between percentile of SGA outcome (3^rd, 5^th and 10^th) and trimester and dosage of tobacco exposure.

Results: Hypertension and smoking appeared to work synergistically to restrict fetal growth. Women who smoked moderately in all three trimesters and had hypertension were 4.34 times more likely to give birth to a 3^rd percentile SGA infant compared to non-smoking normotensive women. However, the effect of hypertension was protective for giving birth to a 10^th percentile SGA infant among women who smoked moderately across all three trimesters.

Conclusions: The timing and duration of tobacco exposures mediate the risk and severity of fetal growth restriction. This relationship was modified by presence of hypertension which may indicate different etiologies between pathologically growth restricted infants and constitutionally growth restricted infants.