Does timing of multivitamin supplementation modify the association between smoking during pregnancy and birth weight?

Study results consistently show that babies born to women who smoke during pregnancy are approximately 200 grams (g) lighter than babies of women who do not smoke during pregnancy. However, it is unclear whether multivitamin supplementation can counteract the adverse effects of smoking during pregnancy. We investigated the interaction between smoking and multivitamin supplementation in pregnant mothers on mean birth weight (BW). Data were collected via telephone interviews by nurse interviewers for mothers of singleton births (n=869). Women who smoked 5+ cigarettes/day throughout pregnancy were considered exposed. Initiation of multivitamin supplementation was categorized as early (before or during the 1st lunar month [LM]), 2nd LM, 3rd LM, 4th LM, later or never, and intermittent. Mean BW was modeled using multiple regression as a function of the dual exposure of smoking and initiation of multivitamins, adjusting for gestational age and mother's age, education, pre-pregnancy body mass index, gravidity, alcohol consumption, family income, marital status, and pregnancy intention. Overall, 7% of mothers smoked during pregnancy and 29%, 38%, 15%, and 5.8% of mothers started multivitamin use early or in the 2nd LM, 3rd LM, or 4th LM, respectively. Babies of smokers had an adjusted mean BW 104g lighter than non-smokers without accounting for multivitamin supplementation. The BW deficit decreased for women whose multivitamin supplementation began in LM3 by 37g, but remained lower for smokers with earlier or later use. Our results suggest that multivitamin supplementation in particular time periods may be beneficial for women who continue to smoke during pregnancy.