Dynamic changes in maternal and fetal calcium metabolism during pregnancy were investigated by simultaneously measuring serum or urinary concentrations of calcium and calcium regulating hormones. Serum concentrations of total calcium in maternal serum decreased significantly, but those of ionized calcium decreased slightly but not significantly late in pregnancy. Maternal serum levels of parathyroid hormone (PTH) were almost the same as non-pregnant values throughout pregnancy, but those of 1 alpha, 25-(OH)2 vitamin D3 increased as pregnancy progressed. Serum levels of calcitonin (CT) in maternal serum increased late in pregnancy but were statistically not significant. Calcium concentrations in maternal urine during pregnancy showed a slight decrease. It is suggested that calcium absorption in the maternal intestine might be increased by the action of increased serum 1 alpha, 25-(OH)2 vitamin D3, and the maternal bone during pregnancy might be kept at the same density as in non-pregnant women because increased CT protects the maternal skeleton by resisting the bone-resorbing activities of 1 alpha, 25-(OH)2 vitamin D3. The concentrations of ionized calcium and CT in umbilical cord blood were higher, but those of PTH and 1 alpha, 25-(OH)2 vitamin D3 were significantly lower than those of the maternal blood at term. It is considered that an active transport mechanism may be involved in the transplacental supply of calcium, and calcium transport from mother to fetus results in a decrease in the calcium concentrations of the maternal serum. Calcium transported into the fetus may be used as fetal body composition such as accumulation in the bone mainly by the action of serum CT.