Maternal-fetal transfer of ionized serum magnesium during the stress of labor and delivery: a human study.

OBJECTIVE

The purpose of this study was to compare levels and fractions of ionized magnesium in maternal venous serum with those in umbilical venous and arterial serum and to determine whether the maternal levels and fractions change during the stress of labor.

METHODS

Utilizing an ion-selective electrode, we determined levels and fractions of ionized magnesium (IMg2+) and levels of ionized calcium (ICa2+) in the maternal venous serum (MVS) of 12 parturients on admission and at the end of labor, as well as in the umbilical venous (UVS) and umbilical arterial serum (UAS) at delivery. A paired-sample study design was used.

RESULTS

Whereas mean levels of ICa2+ did not change significantly (p > 0.05) during labor, the mean (+/- SE) MVS levels of IMg2+ and total magnesium (TMg) fell from 0.50 +/- 0.01 and 0.80 +/- 0.02 mmol/L, respectively, on admission to 0.46 +/- 0.01 and 0.68 +/- 0.01 mmol/L (p < 0.01 and p < 0.001, respectively) at delivery. The ionized fraction, expressed as a percent (IMg2+/TMg x 100), increased from 62.8 +/- 2.1% to 67.8 +/- 1.2% (p < 0.05). In the UVS, the mean IMg2+ level (0.52 +/- 0.02 mmol/L) and the mean ionized fraction (73.6 +/- 1.7%) were higher than in MVS on admission or at delivery (p < 0.05 for all comparisons). The mean IMg2+ level in UAS (0.50 +/- 0.02 mmol/L) was lower than in UVS (p < 0.05), but higher than in MVS at delivery (p < 0.01). Finally, there were significant positive correlations between levels of magnesium (Mg) in MVS and in the UAS or UVS.

CONCLUSIONS

The observation that UAS levels of IMg2+ and TMg were similar to the MVS levels on admission despite the fall in maternal levels during labor points to the presence of homeostatic mechanisms in the fetus and placenta. It is possible that the presence of a higher fraction of unbound, free magnesium in UVS enhances magnesium transport to the fetus and thus homeostasis. Finally, we hypothesize that the fall in the levels of the biologically active form of Mg during labor may be yet another manifestation of the known stress responses to labor.