TUESDAY, March 19 (HealthDay News) -- Women's vitamin D levels during pregnancy do not affect their children's bone health later in life, a new study shows.

British researchers measured vitamin D levels in nearly 4,000 women during pregnancy and assessed the bone mineral density of their children when they were about 12.

Bone mineral density is a measure of bone health. A lower mineral content is associated with poorer bone health and a higher risk of diseases such as rickets.

There was no significant association between a mother's vitamin D levels during pregnancy and her child's bone mineral density, according to the study published online March 18 in The Lancet.

The University of Bristol researchers also found that mothers' vitamin D levels tended to be lowest during the first trimester of pregnancy, and then increased as the pregnancy progressed. As expected, vitamin D levels tended to be lower during the winter and higher during the summer. The skin produces vitamin D in response to sunlight and there are fewer daylight hours in winter than in summer.

The study also said that nonwhite mothers and those who smoked during pregnancy tended to have lower vitamin D levels, but this did not appear to have any affect on their children's bone health.

The researchers said their findings suggest that U.K. health guidelines may be overstating the importance of vitamin D supplementation during pregnancy. Those guidelines recommend that all pregnant and breast-feeding women should take a 10 microgram vitamin D supplement every day.

Vitamin D helps keep bones and teeth healthy. Previous studies into the effects of pregnant women's vitamin D levels on children's bone health have been inconclusive, but this new study is more than 10 times larger than previous studies combined, according to a journal news release.

"Suggesting to pregnant women that their child's future bone health depends on their pregnancy vitamin D status or that by taking supplements they will improve [their child's future bone health], ... I think our study challenges that [suggestion]," study leader Debbie Lawlor said in the news release.

In an accompanying editorial, Philip Steer, of Imperial College London, wrote: "In view of the inconsistency in results [of previous studies], it might seem unclear why vitamin D supplementation is officially recommended for all pregnant and breast-feeding women."

He added: "The safest approach is probably routinely to supplement pregnant women at greatest risk, as defined by the (U.K.) guidelines." These include women of South Asian, black African, black Caribbean or Middle Eastern origin, as well as women with little exposure to sunlight or who were obese before pregnancy, he noted.

Steer added that pregnant women whose diets included few sources of vitamin D, such as oily fish, eggs, meat or fortified margarine or breakfast cereal, might also benefit from supplementation.

Other research has suggested a connection between vitamin D levels in pregnancy and infants' birth weight.

More information

The Harvard School of Public Health has more about vitamin D and health.