Is the association of maternal smoking and pregnancy-induced hypertension dependent on fetal growth?

Objective: The risk of pregnancy-induced hypertension (PIH) is decreased by smoking, but the mechanisms remain unclear. Our objective was to determine whether this association is dependent on decreased fetal growth. Methods: A population-based, retrospective cohort study in the United States was performed consisting of nulliparous women who delivered a singleton birth (n = 8,025,295) between 1995 and 2002. Fetal growth was defined as birthweight for gestational age and characterized as less than 1, 1-2, 3-4, 5-9, 10-19, ..., 90 or greater centiles. Risk and relative risk of PIH before and after adjusting for confounders were estimated. Results: Smoking was associated with decreased risk of PIH with up to a 46% decreased risk of PIH for growth-restricted babies (less than the 10th centile). This association, however, decreased with increasing birthweight centile and was nonsignificant at 20th or greater centile among heavy smokers, at 60th or greater centile for moderate, and 80th or greater centile for light smokers. Conclusion: Smoking was primarily associated with decreased risk of PIH among growth-restricted babies.