Taking vitamin D supplements during pregnancy, as recommended for all women under UK guidelines, has no significant effect on the bone density of babies, according to the first randomised controlled trial of its kind, published in The Lancet Diabetes & Endocrinology journal. However, the findings suggest vitamin D 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.

Bone growth in the womb and in early infancy is linked to overall bone strength later in life, and weaker bones in infancy and childhood may predispose to conditions such as osteoporosis in older age. 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 [1] vitamin D to improve babies' bone health. Although the link is suspected, strong evidence from randomised trials is lacking. The Maternal Vitamin D Osteoporosis Study (MAVIDOS) is the first randomised, placebo-controlled trial designed to measure the impact of pregnancy vitamin D supplementation on the bone health of babies using bone density scanning.

Between October 2008 and February 2014, Professor Cyrus Cooper and colleagues recruited 1134 women from three UK study sites (Southampton, Oxford, and Sheffield), who were between 14 and 17 weeks pregnant, and had low to normal levels of vitamin D. Half (569) of the women took an oral placebo capsule daily, and half (565) took a 25µg [2] 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 women supplemented with vitamin D compared with those who had taken placebo (mean bone mass 61·6g in vitamin D group vs 60·5g in the control). However, further analysis which took into account the season of birth, showed that babies born during winter months to mothers who had taken vitamin D supplements had greater bone mass than winter babies born to mothers who received the placebo (mean bone mass 63.0g in vitamin D group vs 57·5g in the control).

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. This 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.

Co-author Professor Cyrus Cooper, Professor of Rheumatology and Director of the MRC Lifecourse Epidemiology Unit, University of Southampton, says: "Our unit seeks to develop interventions to improve bone health early in life. We found that vitamin is safe and effective in raising vitamin D levels in mothers whose babies are born during winter months."[3]

Co-author Professor Nicholas Harvey, Professor of Rheumatology and Clinical Epidemiology at the MRC Lifecourse Epidemiology Unit, University of Southampton, added: "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. 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."[3]

Writing in a linked Comment, Professor Ian R Reid, Department of Medicine, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand, says: "In pregnancy, low maternal (vitamin D) concentrations have been associated with gestational diabetes, pre-eclampsia, infants who are small for their gestational age, and lower offspring bone mass. From these associations has arisen the widespread belief that vitamin D deficiency is involved in the causation of a wide range of pathological abnormalities, resulting in advocacy for widespread supplementation. After a decade of publications demonstrating vitamin D's many disease associations, we are now entering a new era in which trials of vitamin D supplementation are being completed. These offer the possibility of determining which associations represent causation, and of guiding clinical practice." Reid concludes 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."