Calcium homeostasis and bone metabolism during pregnancy, lactation, and postweaning: a longitudinal study.

Ten women were followed serially to determine the effect of stages of reproduction on calcium and bone metabolism. The study periods were nonpregnant nonlactating, the end of each trimester of gestation, 3 mo lactation, and postweaning. Comparisons were with nonpregnant nonlactating status for each individual. Fractional calcium absorption (P < 0.0001) and concentrations of 1,25-dihydroxyvitamin D (P < 0.01) were higher in the second and third trimesters. Total urinary calcium was higher during pregnancy and lower postweaning. Parathyroid hormone (PTH) concentrations were higher only postweaning (P < 0.01). Markers of bone turnover increased at the third trimester and during lactation: serum tartrate resistant acid phosphatase and bone specific alkaline phosphatase, and urinary deoxypyridinoline (P < 0.01). Serum procollagen I carboxypeptides increased only in the third trimester (P < 0.01). Bone mineral density by single-photon absorptiometry did not differ by period. We conclude that absorption and urinary excretion of calcium increase during pregnancy whereas bone turnover increases during late pregnancy and lactation; only renal changes consistent with an increase in PTH were seen postweaning.