Elevated calcium, phosphorus, and magnesium retention in pregnant rats prior to the onset of fetal skeletal mineralization

Intestinal absorption and renal excretion rates of Ca, P, and Mg were compared in pregnant and control virgin rats fed a purified diet containing 0.55% Ca, 0.54% P, and 0.067% Mg. Four consecutive balance intervals of 5 days each were examined, beginning on day 1 of gestation. During days 6-10 of gestation, pregnant rats had elevated fractional intestinal absorptions of Ca (29.6 +/- 1.7 versus 20.6 +/- 1.5%), P (65.4 +/- 2.0 versus 59.9 +/- 0.9%), and Mg (54.3 +/- 1.5 versus 42.4 +/- 2.6%). Since urinary excretions of these elements did not change, the pregnant rats retained more Ca, P, and Mg than virgin rats. Fractional intestinal absorptions of these elements during pregnancy were similar to control values during days 1-5 and 11-15 of gestation and then rose for each element during days 16-20 of gestation. Presumably because of an increase in glomerular filtration rate, urinary excretions of Ca and Mg were elevated by 40 and 26% during days 16-20 of gestation. In contrast, urinary P excretion was decreased by 30% during days 11-20 of gestation. Analyses of uterine mineral contents indicated the increased maternal Ca and Mg retentions during pregnancy were balanced by the transfers of these elements to the fetuses. In contrast, pregnancy was associated with a net maternal retention of P. These data are consistent with previous observations of increased maternal skeletal mineralization during early pregnancy before the onset of fetal osteogenesis and subsequent enhanced maternal Ca intestinal absorption concurrent with fetal skeletal mineralization.