NICE says there is insufficient evidence to warrant vitamin D supplements in pregnancy, but experts say they are necessary for many women in the UK.
Belgian researchers speaking at a meeting of the British Endocrine Societies in Birmingham suggested that low vitamin D in pregnancy could cause type-1 diabetes in the child.
Their research in mice has shown that giving vitamin D to mice predisposed to developing the autoimmune condition can prevent type-1 diabetes.
Dr Maggie Blott, a consultant obstetrician and spokeswoman for the Royal College of Obstetricians and Gynaecologists, said: 'What we're concerned about is that, if mothers have low vitamin D in pregnancy, the babies will also have low vitamin D.'
Low vitamin D in pregnancy can also lead to rickets and hypocalcaemia in a child, she said.