Calcium Metabolism in Normal Human Pregnancy

Fifteen normal pregnant women and 9 female controls of similar age underwent mineral balance and calcium kinetic studies during and after pregnancy. 48Ca, a stable isotope, was employed in the pregnant women and specimens were measured by neutron activation analysis. Results were related to stage of pregnancy and demonstrated the expected and well documented retention of nitrogen, phosphorus, and calcium associated with fetal development. In addition, there was a 20% increase in miscible calcium pool by term, and an approximate doubling of both pool turnover and bone mineral accretion rates, both rising progressively with duration of pregnancy. Intestinal calcium absorption was twice usual normal levels from the earliest period studied and remained high throughout pregnancy. Virtually all these changes reverted to normal non-pregnant levels by 3 months after delivery. The importance of maternal adjustments as a component in these changes was emphasized by the presence of significant changes in accretion, turnover, and calcium absorption well in advance of the time when most skeletal mineralization occurs in the fetus and by the partial persistence of elevation of these functions for a few weeks after delivery. High doses of both estrogen and progesterone given to a single normal woman for 7 weeks failed to reproduce the changes observed in actual pregnancy. The basis for these anticipatory maternal adjustments is speculated to be an interaction between placental lactogen, estrogens, and parathyroid hormone.