Calcium, magnesium, phosphorus, and parathyroid hormone interrelationships in pregnancy and newborn infants.

The ability to measure directly human parathyroid hormone (hPTH) and ionized calcium (Ca++) is of fundamental importance in understanding their interrelation in pregnancy and the newborn. hPTH and Ca++ progressively increase throughout pregnancy. Since Ca++ is the direct determinant of hPTH levels, a different "set point" appears to be operative throughout pregnancy. The newborn infant, on the other hand, has higher serum Ca++ levels than its mother, and the infant's plasma hPTH is suppressed to undetectable or low levels. Ca++, total serum Ca, magnesium, and phosphorus are all significantly elevated in the serum of the mother at delivery. An active transport mechanism of all these minerals appears to be operative and lowers the maternal serum levels at the termination of labor in relation to serum levels in the third trimester of pregnancy.