Prenatal vitamins are an important part of care for any mother-to-be, but new research shows that women expecting their babies in the winter may benefit from some vitamin D supplements. The vitamin is important for strong bones.

Researchers found that pregnant women who took vitamin D supplements and who had their babies in the winter had children with greater bone mass when compared to counterparts with babies born at the same time but who did not take the supplements.

"This is an important study which shows that vitamin D supplementation for pregnant women can help increase bone strength in children born in winter months," Stephen Simpson, Director of research and programmes at the charity, Arthritis Research UK, said. "Strong bones in childhood are likely to lead to a reduced risk of osteoporosis and fractures later in life, which are associated with disability and loss of independence. As a charity we are committed to funding research such as this, which can shed light on ways to prevent the development of painful, debilitating conditions like osteoporosis."

During the study, researchers from the Medical Research Council (MRC) Lifecourse Epidemiology Unit, University of Southampton, examined over 1,000 pregnant women from Southampton, Oxford and Sheffield in a randomized, double-blind, placebo-controlled trial on vitamin D supplements in pregnancy. The participants either received 1,000 units (25 microgrammes) of vitamin D each day or a placebo capsule from 14 week's gestation until the arrival of their baby.

Findings showed that over 80 percent of women who had received the supplement showed satisfactory levels of vitamin D when measured late in their pregnancy and when compared to just about 35 percent of those in the placebo group. Researchers did not, at first, either find a difference in bone mass between babies born to mothers who took the supplement and those who received the placebo. However, further analysis showed that particularly among women whose babies were born in the winter, those who took the vitamin D supplements during the study period had babies with a mean bone mass of 63 g compared to about 58 g of those in the control group who received the placebo.

The researchers said they believe that better bone mass starting at birth will likely lead to stronger bones in older age. Furthermore, this could also reduce the risk of broken bones as a child and as an adult. 

The study is published today in the journal Lancet Diabetes and Endocrinology.