Taking vitamin D supplements during pregnancy does not significantly improve the bone health of babies, though there may be some seasonal effect, according to UK researchers.
Despite being recommended for all pregnant women under UK guidelines, taking vitamin D supplements was found to have no significant effect on the bone density of babies.
“Mothers’ levels of vitamin D tend to drop from summer to winter”
However, some seasonal effect was identified by the trial. The findings suggested the supplements may be beneficial for babies born in winter months, potentially counteracting the seasonal drop in vitamin D levels caused by a lack of sunlight.
Previous observational studies have associated higher maternal pregnancy vitamin D levels with increased offspring bone mass.
Current UK government guidelines recommend that all pregnant women take daily supplements of 10µg vitamin D to improve babies’ bone health.
Although the link is suspected, strong evidence from randomised trials is lacking, said the authors of the new study journal The Lancet Diabetes & Endocrinology.
Vitamin D supplements during pregnancy questioned
The Maternal Vitamin D Osteoporosis Study (MAVIDOS) is the first randomised controlled trial to measure the impact of pregnancy vitamin D supplementation on the health of babies using bone density scanning.
Researchers recruited 1,134 women from Southampton, Oxford and Sheffield, who were between 14 and 17 weeks pregnant, and had low to normal levels of vitamin D.
Half of the women took a placebo daily and half took a 25µg vitamin D capsule daily until delivery of the baby.
Overall, the researchers found that there was no significant difference in bone mass between the babies born to supplemented women, compared with those who had taken placebo. The mean bone mass was 61.6g in the vitamin D group versus 60.5g in the control group.
However, further analysis which took into account the season of birth, showed babies born during winter to mothers who had taken supplements had greater bone mass than winter babies born to placebo mothers – 63g in vitamin D group versus 57.5g.
“Our advice remains the same and in line with the most recent NICE guidance”
Furthermore, in mothers who gave birth in winter, vitamin D concentrations fell from 14 to 34 weeks gestation in the placebo group, but rose in the treatment group.
The trend suggests that vitamin D supplementation may counteract the drop in the body’s normal levels of vitamin D caused by lack of sunlight when late pregnancy occurs in the winter months, said the study authors.
Co-author Professor Nicholas Harvey, professor of rheumatology and clinical epidemiology at the University of Southampton, said: “Babies’ bones strengthen during the last stages of pregnancy.
“The MAVIDOS trial 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,” he added.
Janet Fyle, professional policy advisor at the Royal College of Midwives, said: “Our advice remains the same and in line with the most recent NICE guidance – risk groups such as children, pregnant women and breastfeeding mothers should take vitamin D supplementation.”
“Midwives come into contact with women from the earliest stages of their pregnancy and are involved with and throughout antenatal care and birth, this makes them key people to talk to women about vitamin supplementation and dietary issues in pregnancy,” she added.
The study was funded by Arthritis Research UK, Medical Research Council, Bupa Foundation, and National Institute for Health Research.