Vitamin D homeostasis in the perinatal period: 1,25-dihydroxyvitamin D in maternal, cord, and neonatal blood.

To investigate vitamin D homeostasis in term pregnancy, we measured 1,25-dihydroxyvitamin D (1,25(OH)2D) in serum samples from 19 term pregnant women and in samples from the placental veins of their infants. Samples were obtained from 14 neonates at 24 hours of age. At delivery, maternal concentrations of 1,25(OH)2D were elevated above normal adult values; placental-vein concentrations in the infants were significantly lower than adult normal or maternal values and bore no relation to maternal values. By the time the infants reached 24 hours of age, their serum concentrations had reached normal adult values, concomitant with a decrease in serum concentration of ionized calcium. We speculate that low 1,25(OH)2D concentrations in utero suggest that there is no need for intestinal calcium absorption in the fetus. Postnatal increase of 1,25(OH)2D may result from its production as a prerequisite to extrauterine requirements for intestinal absorption of calcium and phosphorus.