Major Finding: Normal-weight women who took multivitamins in the periconceptional period were 16% less likely to give birth preterm than were women who did not take them; the benefit was due to a reduced risk of preterm birth after spontaneous preterm labor.

Data Source: An observational study of 27,259 women with singleton pregnancies enrolled in the Danish National Birth Cohort.

Disclosures: Dr. Catov reported that she had no relevant conflicts of interest.

SEATTLE — Use of multivitamins around the time of conception may protect against preterm birth, but the benefit depends on a woman's weight and the type of preterm birth, according to findings of a cohort study among more than 27,000 Danish women.

Normal-weight women were 16% less likely to give birth preterm if they took multivitamins in the periconceptional period. This reduction was due to a lower risk of preterm birth after spontaneous (idiopathic) preterm labor. In contrast, overweight women were not less likely to have a preterm birth if they took multivitamins periconceptionally. Also, use did not reduce the risk of preterm births that were medically indicated or that occurred after premature rupture of membranes (PROM).

“Our data suggest that multivitamin use around the time of conception and implantation may reduce risk of idiopathic preterm labor among normal-weight women,” lead investigator Janet M. Catov, Ph.D., said at the meeting. The dose of vitamins may have been insufficient in overweight women or perhaps they had higher levels of systemic inflammation, she speculated. “As we better understand the complexity of overweight and obesity in pregnancy, I hope that we might better understand that.”

“The fact that we did not see a relationship to membrane rupture suggests that what we might be describing are more sort of early and important characteristics of pregnancy that may… set the stage, if you will, for successful pregnancy or less successful pregnancy,” she commented.

Explaining the rationale for the study, Dr. Catov, assistant professor of epidemiology at the University of Pittsburgh, noted that women's nutritional status is likely important for placentation, and abnormal placentation has been associated with both spontaneous and medically indicated preterm births. “Previous work by our group and others has suggested that periconceptional multivitamin use is related to reduced risk for preeclampsia, early preterm birth, and growth restriction.”

The investigators analyzed data from the Danish National Birth Cohort, which enrolled pregnant women in Denmark between 1997 and 2003.

Analyses were based on women who were recruited at more than 5 weeks' but less than 24 weeks' gestation of a singleton pregnancy, provided detailed information on vitamin use in the periconceptional period (extending from 4 weeks before the last menstrual period to 8 weeks after), and had a live birth.

To look more closely at the issue of timing of multivitamin use, they subdivided the periconceptional period into a preconceptional period (4 weeks before last menstrual period to 2 weeks after) and a postconceptional period (2 weeks after last menstrual period to 8 weeks after). To look at the issue of frequency of multivitamin use, within each 6-week period, they classified use as partial (3 weeks or less) or regular (4 weeks or more).

A total of 19,677 women reported at least some use of multivitamins periconceptionally, and 7,582 did not report any use. Users were somewhat less likely to be younger than age 25 than were nonusers (13% vs. 19%, respectively), to be overweight, defined as having a prepregnancy body mass index of 25 kg/m

Overall, in adjusted analyses, multivitamin users were significantly less likely than nonusers to give birth preterm, meaning before 37 weeks' gestation (hazard ratio, 0.88). But after stratification by weight, this benefit was seen only among women who were of normal weight, defined as having a prepregnancy BMI of less than 25 kg/m

Timing and frequency of multivitamin use were important, according to Dr. Catov. Normal-weight women were significantly less likely to have a preterm birth if they partially used multivitamins preconception and regularly used them post conception (HR, 0.77) or regularly used them in both periods (HR, 0.82). However, they did not have a significant reduction in risk if they only used them regularly post conception or used them partially in both periods.

“Future studies are needed to determine the actual nutrients that might be involved … and to really understand what mechanisms might be involved,” she concluded. “We also need to better understand the relationship between periconceptional multivitamin use and adverse events, and that work is actually under way with our colleagues in Denmark.”

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The effect was limited to idiopathic preterm labor in normal-weight women.

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Periconceptional Multivitamin Use May Prevent Preterm Birth