Seven out of every 10 pregnant women in the United States are not getting enough vitamin D according to a study published in the May issue of the American Journal of Obstetrics and Gynecology. While prenatal vitamins raise vitamin D levels during pregnancy, the study shows that higher doses may be needed for many women.

Adit Ginde, M.D., M.P.H., assistant professor at the University of Colorado School of Medicine and lead author of the study said, "We already know vitamin D is important for bone health of the mother and infant, but we are just starting to scratch the surface about the many potential health benefits of vitamin D during pregnancy."

The latest study shows that many pregnant women in the U.S. have insufficient vitamin D levels. For those women, prenatal vitamins do not provide enough vitamin D and higher doses are needed to raise levels. Women with darker skin, those who cover their skin for religious or cultural reasons and those living further north during winter months are at particularly high risk for lower vitamin D levels.

"Prenatal vitamins do help raise vitamin D levels, but many women start taking them after becoming pregnant," Ginde said. "Although research is ongoing, I think it's best for women to start a few months before becoming pregnant to maximize the likely health benefits."

A growing body of evidence indicates that vitamin D levels have fallen below what's considered healthy in the overall population – likely from decreased outdoor activity. And it has re-emerged as an important nutritional factor in maternal and infant health. Vitamin D deficiency early in life has been linked to increased risk of childhood wheezing and respiratory infections. Lower levels in adults have been linked to cardiovascular disease and cancer.

The study found that some women have enough vitamin D; Ginde cautions that there might be risks from excessive vitamin D intake.

"We need more data from clinical trials of vitamin D supplementation in pregnant women," Ginde said. "If the ongoing trials continue to show benefit, the best strategy will likely be measuring vitamin D levels through a simple blood test and choosing supplementation doses according to those levels. This tailored approach is common in preventive care for people with high cholesterol, and safer and more effective than a one-size-fits-all solution."

The research was supported by the National Institutes of Health. The study team from the University of Colorado School of Medicine, Massachusetts General Hospital and Children's Hospital Boston analyzed nationally representative data collected by the Centers for Disease Control and Prevention from 928 pregnant and 5,173 nonpregnant women of childbearing age.