Association of periconceptional multivitamin use and risk of preterm or small-for-gestational-age births.

The authors' objective was to determine the relation between periconceptional multivitamin use and the risk of small-for-gestational-age (SGA: <5th percentile; 5th-<10th percentiles) or preterm (<34 weeks; 34-<37 weeks) births. Women in the Pregnancy Exposures and Preeclampsia Prevention Study (1997-2001) reported at enrollment their regular multivitamin use in the past 6 months (n=1,823). Women were classified as users or nonusers in multinomial logistic models. After adjustment for race, age, education, enrollment gestational age, and household density, periconceptional multivitamin use was associated with a reduced risk of preterm births (<34 weeks) (odds ratio (OR)=0.29, 95% confidence interval (CI): 0.13, 0.64) and spontaneous preterm births (<34 weeks) (OR=0.40, 95% CI: 0.16, 0.99). Risk of SGA (<5th percentile) was marginally lower (OR=0.64, 95% CI: 0.40, 1.03) after adjustment for smoking, education, parity, enrollment gestational age, and body mass index. Prepregnancy body mass index modified this relation. Nonobese users had a reduction (OR=0.54, 95% CI: 0.32, 0.91) in risk of SGA (<5th percentile); there was no effect among obese women. There was no effect of multivitamin use on risk of preterm births (34-<37 weeks) or SGA (5th-<10th percentiles). Sensitivity analysis for unmeasured confounding by folate intake supported these findings. Study results indicate lower rates of severe preterm births and extreme SGA in women who report periconceptional vitamin use, although these should be considered cautiously until replicated.