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.


METHODS
A cross sectional study design.


METHODS
An urban community hospital obstetrics clinic.


METHODS
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.


METHODS
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.


METHODS
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.
Handwerker
S M
SM
Division of Maternal Fetal Medicine, New York Hospital Medical Center of Queens, Flushing, USA.
Altura
B T
BT
Altura
B M
BM
D001798:Blood Proteins / Q000032:analysis; D002413:Cations, Divalent; D003710:Demography; D004566:Electrodes; D004573:Electrolytes / Q000097:blood*; D005260:Female; D005865:Gestational Age; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000097:blood*; D011247:Pregnancy / Q000097:blood*
D001798:Blood Proteins; D002413:Cations, Divalent; D004573:Electrolytes; D008274:Magnesium