Vitamin D helps maintain calcium levels in the body to keep bones, teeth and muscles healthy.

A number of studies also suggest that taking vitamin D supplements may also help protect against heart disease, cancer, respiratory infections and asthma, as well as conditions related to pregnancy, such as pre-eclampsia and gestational diabetes.

But advice on supplementation to date is conflicting, say Canadian researchers.

To investigate the evidence researchers from the Hospital for Sick Children in Toronto reviewed 43 randomised controlled trials involving more than 8000 women.

Their study, published in The British Medical Journal (The BMJ), assessed the effects of taking vitamin D supplements during pregnancy on various maternal and child outcomes.

The results show that taking supplements during pregnancy increased vitamin D levels in both the mother's bloodstream and umbilical cord blood.

But the researchers did not consistently find that higher doses of vitamin D led to healthier women and babies.

They found that vitamin D did increase the average birth weight of a baby by 58g, and reduced the risk of babies being born small.

But more detailed analysis weakened the statistical significance of these findings.

There appeared to be no effect on whether or not babies were born before their due date.

The researchers did find that mothers who took vitamin D supplements in pregnancy were less likely to have children with a wheeze when they were aged three.

Due to the inconclusive evidence on the effectiveness of taking vitamin D supplements during pregnancy the issue "will probably remain unanswered in the foreseeable future", the authors said.

"Cautious projections for the next decade suggest that we will eventually know more about vitamin D in pregnancy than we do now, but in the absence of a coordinated effort and funding to conduct large new trials, some of the most critical questions about the effectiveness of prenatal vitamin D supplementation will probably remain unanswered in the foreseeable future," they concluded.

The authors have called for new larger trials on the vitamin's use in pregnancy to measure the health outcomes.