The concentration of free calcium ions and total calcium in pregnancies at term.

The concentration of free calcium ions (cCa2+), total calcium (ctCa) and pH was measured in 46 normal pregnancies at term. Measured with a combined Ca2+-pH electrode system in whole blood, maternal plasma cCa2+ was 1.18 +/- 0.03 mmol/l, and the corresponding plasma pH was 7.37 +/- 0.05 (mean +/- SD). In the cord vein, plasma cCa2+ was 1.44 +/- 0.07 mmol/l and pH 7.37 +/- 0.05. In the cord artery, plasma cCa2+ was 1.47 +/- 0.07 mmol/l and pH 7.29 +/- 0.07.