Taking a vitamin D supplement during pregnancy may strengthen the bones of babies born in the winter, a new study has found.

The study, Maternal Vitamin D Osteoporosis Study (MAVIDOS), was published online on March 1st. It is the first randomized controlled trial of its kind, investigating the efforts of bone mass in offspring, and how it relates to maternal vitamin D supplementation.

1,134 women from Southampton, Oxford and Sheffield participated in the study. All were between 14 and 17 weeks pregnant, and had low to normal levels of vitamin D. Half took a 25 microgram vitamin D capsule daily while the other half were given a placebo pill.

The researchers found no significant difference in bone mass between the babies born to each group of mothers. But further analysis showed that those babies born during the winter months had higher bone mass than those whose mothers received the placebo.

“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 Professor Nicholas Harvey, co-author of the study.

“The 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.

Current UK guidelines recommend that all pregnant women take a daily vitamin D supplement of 10 micrograms.

“The results also suggest that those women due to deliver in the winter months may consider taking a higher dose, at 1000 IU per day,” added Harvey.

Dr Benjamin Jacobs, consultant paediatrician at the Royal National Orthopaedic Hospital in London, believes this may open conversations on current vitamin D recommendations in the United Kingdom.

“This study therefore needs to be taken into account when reviewing the current vitamin D advice, particularly for pregnant women in the UK and beyond.”