OBJECTIVE: Little is known about ion regulation in fetuses. Our aim was to determine the effects of magnesium sulfate therapy on ionized (bioactive) magnesium in the cord blood of pregnancies complicated by preeclampsia. STUDY DESIGN: Seventy-four pregnant women were studied (37 preeclamptic and 37 controls matched for maternal age, gravidity, and gestational age). The preeclamptic women received intravenous magnesium sulfate 6 gm load followed by 2 gm/hour for ≥4 hours; controls were not preeclamptic and received no magnesium. Maternal venous and fetal cord blood samples were obtained from study and control patients and were analyzed for sodium, potassium, total magnesium, ionized magnesium, total calcium, and ionized calcium. Comparisons between the groups were made and analyzed by the Mann-Whitney U test. RESULTS: There were no significant differences between the treatment and control group cord samples with respect to sodium or potassium. However, total magnesium and ionized magnesium were significantly elevated (p < 0.001) in cord samples of the treated group. At the same time ionized calcium and total calcium were reduced. Interestingly, ionized calcium levels were lower in preeclamptic women before magnesium sulfate therapy was begun, whereas total calcium levels were not different. Importantly, there was no difference between maternal and fetal ionized magnesium levels in either treatment or control groups. CONCLUSIONS: In preeclamptic women undergoing magnesium sulfate therapy, ionized magnesium levels in cord blood parallel maternal levels. Before magnesium therapy ionized calcium levels were lower in preeclamptic women than in matched controls. In the presence of elevated magnesium levels ionized calcium appears to be tightly regulated. (Am J Obstet Gynecol 1996;175:213-7.)