CALCIUM HOMEOSTASIS DURING PREGNANCY AND LACTATION IN PRIMIPAROUS ANDMULTIPAROUS WOMEN WITH SUB-ADEQUATE CALCIUM INTAKES

Low calcium intake and parity may be factors affecting calcium homeost asis during pregnancy and lactation. In this study, changes of urinary indices related to bone and renal adjustments of calcium homeostasis were measured in each trimester of pregnancy and in the lactation peri od, in primiparous (n = 35) and multiparous (n = 25) women with habitu al calcium intakes about half the RDA for this nutrient, in comparison to never-pregnant (control) women (n = 23) with similar calcium intak es. Calcium and phosphorous urinary excretion did not change during pr egnancy but decreased in the lactation period, similarly in primiparas and mutiparas (p < 0.01). Calcium urinary excretion was not different between pregnant and control women, indicating that renal conservatio n may contribute to calcium homeostasis during pregnancy with sub-adeq uate as opposed to adequate calcium intakes. In lactation, urinary cal cium excretion was 32% lower than in the controls, indicating renal ca lcium conservation also in this period, Hydroxyproline urinary excreti on increased during pregnancy and decreased in the lactation period, b oth in primiparas (p < 0.001) and multiparas (p < 0.05). Excretion was , on average, 152% and 80% higher than excretion in the control women, in the third trimester of pregnancy and in lactation, respectively, c onsistent with increased bone mobilization in these periods. Consideri ng all pregnant women, hydroxyproline excretion in the third trimester correlated weakly but significantly with pregnancy weight gain (r = 0 .275, p = 0.04). Multiparity did not affect the renal handling of calc ium and phosphorous during pregnancy and lactation, neither hydroxypro line excretion in lactation. However, hydroxyproline excretion in the third trimester of pregnancy was 58% higher in primiparas compared to multiparas, in spite of similar weight gains during pregnancy. This co uld be due to differences between primiparas and multiparas in the com position of tissue gained during pregnancy or to an adaptive mechanism in multiparas to protect maternal bone from excessive calcium loss du ring pregnancy.