Vitamin D metabolism during pregnancy.
Vitamin D metabolism is altered in the pregnant animal, presumably in response to fetal demands for calcium. Circulating levels of 1,25-dihydroxyvitamin D are elevated in the pregnant animal. The stimulus of this increase and the hydroxylase(s) (placental or renal) responsible are unknown. Maternal plasma 25-hydroxyvitamin D levels have been reported to be both unchanged and decreased during pregnancy but very much dependent upon exposure to ultraviolet light and vitamin D supplementation. The major vitamin D metabolites (25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D) circulate in fetal plasma but generally at lower concentrations than in the mother (exception is the sheep). All of these metabolites are able to cross the placenta. The fetal kidney and placenta both have 25-hydroxyvitamin D1 alpha- and 24-hydroxylase activity. However, the relative contribution of mother, fetus, and placenta to fetal vitamin D metabolism has yet to be fully determined.