Women who take multivitamins regularly around the time they get pregnant appear to have a lower risk of going into labor prematurely or having a smaller-than-normal baby.
That's according to a study of nearly 36,000 pregnant Danish women, who were asked about their diet, weight and vitamin use, among other things.
Poor nutrition is thought to play a role in pregnancy complications, such as preterm births and poor growth rates within the womb.
The new study strengthens that link, but it doesn't prove that taking multivitamins is a good idea for women who plan to get pregnant or already are, researchers warn in the American Journal of Clinical Nutrition.
In fact, U.S. health officials advise expectant mothers against taking regular vitamins, which might harm the baby. But they do recommend supplementing the diet with folic acid, which cuts the chance of certain birth defects.
The new work looked at multivitamin use around the time of conception - four weeks before and eight weeks after a woman's last period - which hasn't been studied much.
Among women who said they had taken multivitamins at least eight out of the 12 weeks, there were 4.3 per cent preterm births (before 37 weeks). For those who didn't take the supplements, the number was 5.3 per cent.
The vitamin-popping women were also less likely to have a smaller-than-normal baby.
Those links held even after accounting for differences between the two groups - such as diet and smoking - but only in normal-weight women. The reasons aren't clear, but could be linked to problems in absorbing the nutrients, suggest Janet M. Catov, of the University of Pittsburgh, and her colleagues.
Still, they stop short of recommending that women start taking multivitamins when trying to get pregnant.
First, women who took the supplements appeared to be healthier in the first place, and there is no perfect way to prevent that from muddying the findings.
And second, there is a dearth of studies to test the effects of vitamins on babies' health.