OBJECTIVE To compare serum levels of ionized and total magnesium with those of ionized calcium, total calcium, sodium, and potassium over the course of pregnancy in normal women and in women who develop preeclampsia. METHODS We collected venous serum samples from 31 pregnant women during their first, second, and third trimesters. Gestational ages ranged from 6 to 37 weeks. Samples were analyzed for ionized and total magnesium, ionized and total calcium, sodium, and potassium using a biomedical chemistry analyzer. Data were analyzed with repeated-measures analysis of variance and two-way repeated-measured analysis of variance. RESULTS In 22 normal pregnant women, both serum ionized and total magnesium levels decreased significantly with increasing gestational age. No changes in sodium, potassium, or ionized or total calcium were observed. Nine of the 31 subjects developed preeclampsia by term; serum total magnesium levels decreased significantly by the second trimester in these women compared with those of normal pregnant women. CONCLUSIONS Our results provide evidence of decreases in ionized and total magnesium levels with increasing gestational age during normal pregnancy, as well as evidence of a magnesium disturbance in women who later develop preeclampsia. Future studies of magnesium balance in women at risk for developing complications of pregnancy are indicated.