A recent study of 1,823 women published in the American Journal of Epidemiology found that women who take multivitamins during the 6 months prior to getting pregnant have a lower risk of preterm (less than 34 weeks’ gestational age) and small-for-gestational-age (SGA) births (less than the 5th percentile). Regular multivitamin use was reported by 47 percent of the women enrolled in the Pregnancy Exposures and Preeclampsia Prevention study.

Infants born to multivitamin users were less likely to be born preterm (prior to 34 weeks’ gestation), with a reported odds ratio of 0.29, signifying only about one-third the risk compared to non-multivitamin users. Small-for-gestational age births (below the 5th percentile) were also reduced in multivitamin users, with only about two-thirds the risk (odds ratio=0.64) when compared to non-multivitamin users and after controlling for smoking, education, parity, prepregnancy body mass index, and baseline gestational age.

No difference in preterm births between 34 and 37 weeks and SGA births between the 5th and 10th percentiles was observed between the two groups. So it appears that periconceptional multivitamin use is most protective for preventing births prior to 34 weeks’ gestational age and SGA births falling below the 5th percentile.

Additionally, nonobese women who used multivitamins had about half the risk of an SGA birth below the 5th percentile (odds ratio=0.54) when compared to nonobese women who did not use multivitamins. No difference was found in SGA births between obese women who used or did not use multivitamins in the periconceptional period. One possible explanation for the difference in nonobese women but not in obese women is that the obese women may have larger cellular and tissue stores of fat-soluble vitamins, vitamin B12, and vitamin B6 (two water soluble vitamins that are stored by the body) resulting from a higher energy intake, thus negating the effect of multivitamin supplementation. Nonobese women may have more marginal stores of certain vitamins so that supplementation in the nonobese group might contribute significantly to body vitamin levels.