Taking a high daily dose of vitamin D during pregnancy can significantly reduce a woman's risk of giving birth prematurely, according to a study conducted by researchers from the Medical University of Charleston, South Carolina, and funded by the National Institutes of Health.

"I'm telling every pregnant mother I see to take 4,000 IUs and every nursing mother to take 6,400 IUs of vitamin D a day," said researcher Bruce Hollis. "I think it is medical malpractice for obstetricians not to know what the vitamin D level of their patients is. This study will put them on notice."

Pregnant women in the United Kingdom and the United States are currently advised to take 400 IU of vitamin D per day.

In the first experimental (rather than observational) study into the effect of vitamin D supplementation on the risk of premature birth, researchers assigned 600 pregnant women living in the Charleston area to take either 400 IU or 4,000 IU per day of vitamin D. Levels of vitamin D and calcium in the women's blood and urine were tested once per month to ensure participant safety. The study included roughly equal numbers of white, black and Hispanic participants.

At both 32 and 37 weeks, the rate of premature birth in the 4,000 IU group was half that of the 400 IU group. Significantly fewer "small for date" babies were also delivered to the 4,000 IU group.

In addition, women receiving more vitamin D were less likely to suffer from respiratory, vaginal, gum or other infections. They were 30 percent less likely to suffer from "core morbidities" of pregnancy, such as diabetes, hypertension and pre-eclampsia. Babies born to women in the high vitamin D group had lower rates of colds and eczema than babies in the other group.

There were no adverse effects observed from either vitamin D dose.