To evaluate whether parathyroid hormone-related protein (PTHrP) plays any role in calcium metabolism during pregnancy and the perinatal period, we measured parathyroid hormone (PTH), immunoreactive PTHrP and calcium in maternal serum of women during pregnancy and puerperium, and in maternal and cord serum at delivery. Serum PTH levels in pregnant women were lower than those in nonpregnant women (p < 0.01). However, serum PTHrP levels were similar in the two groups of women. Cord serum total and ionic calcium levels were higher, and cord serum PTH levels were lower compared to maternal values (p < 0.01). Cord serum PTHrP levels were higher than maternal values, and umbilical arterial levels were higher than umbilical venous levels (p < 0.01). Higher levels of PTHrP in umbilical arterial blood than in umbilical venous blood suggest that the main source of cord blood PTHrP may be the fetus. PTHrP is reported to stimulate placental calcium transfer from mother to fetus in animals. The elevated cord blood PTHrP level in association with the decreased PTH level suggests that stimulation of placental calcium transfer by PTHrP may also be operative in humans.