Serum calcium concentrations in human pregnancy.

Total and ionic calcium concentrations were measured in maternal serum from nonpregnant (N=30), pregnant (N=137), and early puerperal (N=17) women. In this cross-sectional study, total calcium levels (measured by atomic absorption spectrophotometry) exhibited the previously described pattern of a decline beginning in the first trimester and continuing until approximately 32 weeks, with a slight terminal rise from thence to term. Mean (+/-SD) values for the three trimesters were 4.80 (+/- 0.13), 4.56 (+/- 0.14), and 4.46 (+/-0.16) mEq. per liter, respectively, with all differences statistically significant. Ionic calcium levels (measured by calcium flow-through electrode) declined to a lesser (though still statistically significant) degree; mean (+/- S.D.) values for the three trimesters were 2.33 (+/-0.07), 2.28 (+/-0.08), and 2.24 (+/-0.06) mEq. per liter, respectively. Because of the differential rates of fall, the proportion of total calcium represented by the ionic component increased progressively during gestation. The results indicate that the major portion of the characteristic decline in maternal serum total calcium concentration with pregnancy reflects changes in the nonionized (albumin-bound) fraction while the ionic portion declines only slightly. The concentration of calcium ions in serum is maintained within a very narrow physiologic range throughout gestation.