There have been no direct measurements of placental mineral transport in human babies. In vitro study of intact placentas as well as isolated trophoblast membranes 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.