Free calcium ion concentration in pregnancy. A longitudinal study on 22 normal pregnant women

302 samples from 22 normal pregnancies were frozen and kept for the analysis of free calcium ion concentration [Ca2+](7.4) at 37 degrees C and pH 7.40, after equilibration with CO2. Individual regressions were performed, and the concentration was found to increase slightly but significantly (5%, p less than 0.005) with the progress of the pregnancy. This increase excludes hypocalcemia as the cause of parathyroid overactivity during pregnancy, and a higher set point of the parathyrin (PTH) secretion is suggested.