Serum ionized magnesium and other electrolytes in the antenatal period of human pregnancy.

OBJECTIVE

To determine: 1) the concentrations and fractions of serum ionized magnesium (IMg2+) in normal antenatal patients and 2) whether they are altered by concentrations of other cations and serum proteins.

DESIGN

A cross sectional study design.

SETTING

An urban community hospital obstetrics clinic.

SUBJECTS

144 normal antenatal patients divided into four gestational age groups: (1) 25 at 6 to 12 weeks, (2) 55 at 13 to 20 weeks, (3) 37 at 21 to 27 weeks, and (4) 27 at 28 to 38 weeks.

INTERVENTIONS

Utilizing ion selective electrodes, we determined concentrations and fractions of ionized magnesium, concentrations of ionized calcium, sodium, and potassium, and the pH level in the venous serum of the patients. Total magnesium (TMg), total calcium, total protein, albumin, and inorganic phosphorus were measured.

MEASURES OF OUTCOME

The mean concentrations and fractions were compared between the groups. Correlations between concentrations or fractions of Mg and concentrations of the other cations or proteins were determined.

RESULTS

All values are means+/-SD. IMg2+ concentrations are 0.53+/-0.03 mmol/L in the first trimester and are lowest (0.49+/-0.05 mmol/L, p<0.001) by the third trimester. The percent of ionized magnesium remains at approximately 66% in each antenatal group. Although mean Mg concentrations fall with advancing gestational age, the main determinant of the IMg2+ concentration in an individual patient is her TMg concentration.

CONCLUSIONS

IMg2+ concentrations and fractions remain within a very narrow range during pregnancy and are minimally affected by physiologic hemodilution. We hypothesize that mobilization of intracellular or bone stores of magnesium helps maintain serum concentrations, thus placing a stress on magnesium balance.