The effectiveness of vitamin D supplements during pregnancy depends on the season and on the mother's weight gain, according to a new study.
The findings, published the Endocrine Society's Journal of Clinical Endocrinology and Metabolism, showed pregnant women respond differently to vitamin D supplementation depending on their individual attributes.
The University of Southampton research suggests that supplements are less effective at raising vitamin D levels in pregnant women if they deliver their babies in the winter, have low levels of the vitamin early in pregnancy or gain more weight during pregnancy.
The scientists say the results show that levels should be tailored according to individual risk factors.
Professor Nicholas Harvey, of the Medical Research Council Lifecourse Epidemiology Unit, University of Southampton, said: "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."
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, foetal development and the child's long-term skeletal health.
More than 800 pregnant women were recruited for the Maternal Vitamin D Osteoporosis Study (Mavidos), and 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.
Professor Cyrus Cooper, director, and professor of rheumatology at the University of Southampton's MRC Lifecourse Epidemiology Unit, said: "Our findings of varied responses to vitamin D supplementation according to individual attributes can be used to tailor approaches to antenatal care."
The study was funded by the charity Arthritis Research UK, with further funding support from the MRC, National Institute for Health Research (NIHR) and the Bupa Foundation.