The placenta actively transports calcium, phosphorus and magnesium against concentration and electrochemical gradients. Studies in lambs and rodents have shown that PTHrP stimulates active placental calcium transport, with PTH having a lesser ability that has been demonstrated in Pth null fetuses and isolated placentalmembranes. Vitamin D, calcitriol, VDR, and calcitonin are not required to stimulate placental calcium transport or regulate placental expression of calcium-
transporting genes and proteins, nor is FGF23 needed for placental phosphorus transport and expression of the relevant phosphotropic genes and proteins. Perfusion of human placentas in vitro has yielded estimates that active transport of calcium comprises about 1/3 of the forward