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But for babies born in the winter, "sunshine vitamin" supplementation may help counteract low levels caused by seasonal loss of sunlight, the researchers said.
Some previous studies have suggested that higher vitamin D in mothers during pregnancy are associated with greater bone density in babies, and guidelines in the United Kingdom recommend vitamin D supplements for all pregnant women.
This study included more than 1,100 British women, 14 to 17 weeks pregnant, with low to normal levels of vitamin D. Half took a 1,000 International Unit vitamin D capsule daily until they had their baby, while the other half took a placebo.
Overall, there were no significant differences in bone mass between babies born to women who took vitamin D supplements and those who took the placebo, researchers reported.
But among babies born in the winter, those whose mothers took vitamin D supplements had greater bone mass than those whose mothers took the placebo.
The researchers also found that among mothers who delivered in the winter, vitamin D levels fell between 14 and 34 weeks of pregnancy in the placebo group, but not in the vitamin D supplementation group.
That suggests that for women in the late stages of pregnancy during winter, vitamin D supplements may counteract the decrease in normal vitamin D levels caused by lack of sunlight, the researchers said.
The study results were published March 1 in The Lancet Diabetes & Endocrinology.
"Babies' bones strengthen during the last stages of pregnancy. Since sunlight is our most important source of vitamin D, mothers' levels of vitamin D tend to drop from summer to winter, and babies born in the winter months tend to have lower bone density than those born during the summer," said study co-author Nicholas Harvey. He is a professor of rheumatology and clinical epidemiology at the University of Southampton.
This study "has given us the first evidence that supplementing mothers with vitamin D during pregnancy counteracts the seasonal drop in maternal vitamin D levels and may help to ensure good bone development in these winter births," Harvey said in a journal news release.
Low maternal concentrations of vitamin D in pregnancy have been associated with gestational diabetes, pre-eclampsia, infants who are small for their gestational age, and lower child bone mass, Ian Reid wrote in an accompanying journal editorial.
As this and other trials of vitamin D supplementation are being completed, "these offer the possibility of determining which associations represent causation, and of guiding clinical practice," said Reid, a professor of medicine at the University of Auckland in New Zealand.
Reid concluded that "in pregnancy and other contexts, we should be moving to targeted supplementation with vitamin D in individuals likely to have [low vitamin D concentrations] and away from mass medication, which is without proved benefit."
-- Robert Preidt
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