Serum magnesium levels in pregnancy and preterm labor.

Pregnancy is marked by a state of hypomagnesemia. The serum magnesium level shows no gestational dependence (mean, 1.79 +/- 0.44 mg/dl) until 33 weeks, at which point it continuously declines. Serum magnesium is not depressed further with the onset of labor at term. Patients in preterm labor have a significantly depressed serum magnesium level (mean, 1.60 +/- 0.46 mg/dl; 21 to 33 weeks; p less than 0.0005). This level was not dependent on whether the etiology for the preterm labor was premature rupture of the membranes (PROM), twin gestation, abruption, placenta previa with bleeding, or chorioamnionitis. With PROM, the serum magnesium level was not depressed prior to the initiation of preterm labor. However, observation of hypomagnesemia for this and other etiologies just prior to the initiation of preterm labor were not available. Possible mechanisms by which hypomagnesemia induces uterine irritability are explored, including inhibition of adenyl cyclase with resultant increase in cytoplasmic calcium levels. Patients with diabetes mellitus appeared to have slightly reduced serum magnesium levels, but the results were not statistically significant. Magnesium levels in patients with preeclampsia were not significantly different from controls. Hypomagnesemia (magnesium 1.4 mg/dl or less) may be a marker for true preterm labor.