For a woman eating a healthy diet, multivitamin supplements during late pregnancy could do more harm than good, a new study suggests.

British researchers found that a woman’s risk of delivering prematurely tripled if she continued taking the prenatal pills into her third trimester.

“These supplements are available over-the-counter in the United Kingdom and frequently promoted as being beneficial for mums-to-be,” Dr. Nigel Simpson of the University of Leeds in the U.K., and one of the authors of the study, told Reuters Health by email.

However, some weaknesses in the study may stand in the way of translating the finding into practice, Dr. James Mills, of the U.S. National Institute of Child Health and Human Development told Reuters Health.

While some studies in developing countries have found prenatal supplements to be beneficial, whether or not it also is in developed countries-where most women are presumably already well-nourished-has not been thoroughly studied.

To fill this void, Simpson and his colleagues assessed the diets and supplement use of nearly 1,300 pregnant women recruited at Leeds Teaching Hospitals between 2003 and 2006.

Overall, slightly more than 4 percent of babies were born weighing less than 2500 grams and categorized as low birthweight. About the same number of babies were born prematurely, defined as before 37 weeks of pregnancy.

The team saw no differences in the risks of having a low birthweight baby for the more than 80 percent of women who took supplements at any point during pregnancy compared with those who took none.

However, the approximately 30 percent of women taking supplements during their third trimester were three times as likely to have a premature delivery, after taking smoking, alcohol consumption and other relevant factors into account.

Why this would be true is unclear. One possibility, according to the authors, is that interactions between different vitamins and minerals led to a reduction in the nutrients available for the growing fetus.

And women in the study were already getting enough of most vitamins and minerals contained in prenatal supplements from their diets, with the exceptions of vitamin D, iron, folate, selenium and iodine, note the authors in the British Journal of Obstetrics and Gynecology.

The U.S. National Institute of Child Health and Human Development’s Mills said that a few weaknesses of the study make its significance less clear. Since the U.K. stops short of officially recommending prenatal multivitamins, British women who chose to take the supplements may have been those who were already at a greater risk for pregnancy problems.

Mills is also concerned that the relationship with premature delivery could have simply appeared by chance, given the large number of comparisons the researchers made between various birth outcomes and supplement use.

The study team acknowledges that larger, more rigorous studies are necessary to confirm their results. For now, Simpson says pregnant women probably don’t need multivitamins past their first three months, after which time they might actually do harm.

“Eating a healthy diet,” he said, “is likely to be sufficient for expectant mums.”

SOURCE: British Journal of Obstetrics and Gynecology, March 29, 2010.