Perfusion of human placentas in vitro has yielded estimates that
active transport of calcium comprises about 1/3 of the forward
flow from mother to fetus. Human trophoblasts express the same
calciotropic and phosphotropic genes and proteins that are expressed
in adult kidneys and intestines, and which are required for active pla-
cental calcium and phosphorus transport.
There have been no directmeasurements of placentalmineral trans-
port in human babies. In vitro study of intact placentas aswell as isolat-
ed trophoblastmembranes have provided some supportive evidence for
the roles of PTHrP and PTH in stimulating placental calcium transport
and gene expression. Functional evidence from humans suggests that
net placental calcium transport is increased when maternal serum
calcium is high, which explains why maternal hypercalcemia causes
abnormal suppression of the fetal parathyroid glands. Conversely,
significant maternal hypocalcemia due to hypoparathyroidism or
pseudohypoparathyroidism may reduce the net placental calcium
flow, and explain why these conditions have provoked compensatory
fetal hyperparathyroidism, parathyroid gland hyperplasia, skeletal demineralization, intrauterine fractures, and bowing of the long bones.