Vitamin D supplements for pregnant women should be customised to the individual to ensure benefits are felt

The findings, published the Endocrine Society's Journal of Clinical Endocrinology & Metabolism, showed pregnant women respond differently to vitamin D supplementation depending on their individual attributes.

The University of Southampton researchers suggest that supplement levels should be tailored according to individual risk factors.

Vitamin D is a hormone that helps the body absorb calcium. It plays a crucial role in bone and muscle health. The skin naturally produces vitamin D after exposure to sunlight but people also obtain smaller amounts of the vitamin through foods, such as milk fortified with vitamin D.

Evidence suggests vitamin D deficiency during pregnancy can harm maternal health, fetal development and the child's long-term skeletal health.

Professor Nicholas Harvey, of the Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, who led the study with Dr Rebecca Moon, Clinical Research Fellow, comments: "It is important for pregnant women to have sufficient levels of vitamin D for the health of their baby. Our study findings suggest that in order to optimise vitamin D concentrations through pregnancy, the supplemental dose given may need to be tailored to a woman's individual circumstances, such as the anticipated season of delivery."

The Maternal Vitamin D Osteoporosis Study (MAVIDOS), is a multi-centre, double-blind, randomised, placebo-controlled trial of vitamin D supplementation in pregnancy. More than 800 pregnant women were recruited and randomised to take either 1000 units (25 micrograms) of vitamin D every day or a matched placebo capsule from 14 week's gestation until delivery of the baby.

Analysis showed that participants who received the vitamin D supplement achieved different levels of vitamin D in the blood, even though they received the same dose. Researchers found women who delivered in the summer, who gained less weight during pregnancy and who had higher vitamin D levels early in pregnancy tended to have higher levels of vitamin D in the blood than their counterparts. Women who consistently took the supplement also had higher levels of vitamin D than participants who did not.

"Our findings of varied responses to vitamin D supplementation according to individual attributes can be used to tailor approaches to antenatal care," said Professor Cyrus Cooper, Director, and Professor of Rheumatology at the MRC Lifecourse Epidemiology Unit, University of Southampton. "This work forms part of a larger programme of research at the MRC Lifecourse Epidemiology Unit, University of Southampton, addressing the early life determinants of bone development, and will inform novel strategies aimed at improving bone health across future generations."