Vitamin D deficiency in pregnancy: what is the evidence?

Pregnancy is a state of increased vitamin D requirements to support fetal development. Vitamin D deficiency is common in pregnant women and has been linked to a wide range of adverse maternal and fetal outcomes. Many epidemiological studies have examined vitamin D status during pregnancy in relation to the health of the mother and baby, but have used study designs where reverse causality may account for the link to disease. Stronger study designs, or meta-analyses of such studies, provide inconsistent results, although very low vitamin D levels are likely to be adverse. Intrauterine exposures can have long-lasting effects into adulthood, so that, while vitamin D deficiency during pregnancy should be avoided, supplementation to high levels may not be beneficial in either the short or long term.