Pregnant women should take vitamin D supplements on a daily basis, say researchers in the UK.
According to the team at University College London Institute of Child Health, official bodies currently offer conflicting advice.
The Department of Health in the UK suggests pregnant women to ensure they receive a certain level of vitamin D - 10 micrograms per day.
The researchers say this in effect endorses use of supplements, because diet and the sun provide too little.
But the National Institute of health and Clinical Excellence (NICE) said in its guidance that it did not support supplements.
It has instead said that women should simply 'be informed' about the importance of having adequate vitamin D levels during their pregnancy and while breastfeeding.
The paper said the UK was the only one of 31 countries examined which did not recommend that women of reproductive age took a vitamin D supplement, and that it also fails to endorse a daily supplement for expectant mothers.
"The incidence of vitamin D deficiency in pregnant women in Britain is unacceptably high, especially during winter and spring," the BBC quoted Dr Elina Hypponen, co-author of the paper, as saying.
"This is compounded by a lack of exposure to sunlight and the limitations of an average diet to meet the optimal need.
"In the most severe cases, maternal vitamin D deficiency can be life threatening to a newborn.
"We believe that the routine provision of a daily supplement throughout pregnancy would significantly decrease the number of mothers who are clearly vitamin D deficient, reducing related serious risks to their babies," Hypponen added.
The researchers reported the paper in the British Journal of Nutrition.