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 no npregnant nonlactating, the end of each trimester of gestation, 3 mo l actation, and postweaning. Comparisons were with nonpregnant nonlactat ing status for each individual. Fractional calcium absorption (P < 0.0 001) and concentrations of 1,25-dihydroxyvitamin D (P < 0.01) were hig her in the second and third trimesters. Total urinary calcium was high er during pregnancy and lower postweaning. Parathyroid hormone (PTH) c oncentrations 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 phosph atase, and urinary deoxypyridinoline (P < 0.01). Serum procollagen I c arboxypeptides increased only in the third trimester (P < 0.01). Bone mineral density by single-photon absorptiometry did not differ by peri od. We conclude that absorption and urinary excretion of calcium incre ase during pregnancy whereas bone turnover increases during late pregn ancy and lactation; only renal changes consistent with an increase in PTH were seen postweaning.