Do North American women need supplemental vitamin D during pregnancy or lactation?

Studies in European and other countries have shown that vitamin D deficiency during pregnancy may adversely affect fetal growth, bone ossification, tooth enamel formation, and neonatal calcium homeostasis. Whether effects of vitamin D deficiency on pregnant or lactating mothers differ from effects observed in nonpregnant or nonlactating women is not clear. Poor maternal vitamin D status during lactation results in low breast-milk vitamin D. However, human milk usually contains small vitamin D amounts and, under normal circumstances, the sunshine exposure of human-milk--fed infants is the major factor affecting their vitamin D status. Mothers at risk of vitamin D deficiency are those who avoid dairy products, which are routinely vitamin D fortified, and live in more northern latitudes. Dark-skinned women also are theoretically at risk of vitamin D deficiency. Sunshine exposure is a major vitamin D source, and given adequate exposure, supplemental vitamin D is not necessary. However, defining adequate sunshine exposure is difficult.