OBJECTIVE To determine the effect of maternal intravenous (IV) magnesium sulfate administration on fetal serum and amniotic fluid (AF) concentrations of magnesium. METHODS Thirty-six patients underwent fetal blood sampling for prenatal diagnosis of one of several abnormal conditions. Fifteen subjects had uterine contractions at the time of funipuncture and received IV magnesium sulfate therapy to quiet the uterus before the procedure. Three groups of subjects were defined: 21 untreated controls, ten women who received magnesium treatment for 1 hour, and five who received it for 3 hours before fetal blood sampling. Magnesium concentrations in maternal and fetal serum and AF were compared among the three groups. RESULTS Patients who received magnesium sulfate for 1 and 3 hours had significantly higher concentrations of serum magnesium (4.11 +/- 0.6 and 5.54 +/- 0.2 mg/dL, respectively) than untreated subjects (1.77 +/- 0.2 mg/dL) (P < .0001). Fetal serum magnesium concentrations were significantly higher in the 1- and 3-hour groups (2.48 +/- 0.1 and 4.44 +/- 0.2 mg/dL, respectively) than in controls (1.67 +/- 0.2 mg/dL) (P < .0001). The AF magnesium concentration was significantly increased only after 3 hours of elevated maternal magnesium levels (1.45 +/- 0.2 and 2.84 +/- 0.2 mg/dL in controls and 3-hour group, respectively; P < .0001). The correlation between maternal and fetal blood magnesium concentrations was highly significant (r = 0.89; P < .0001), as was the correlation between fetal serum and AF concentrations (r = 0.84; P < .0001). CONCLUSIONS Magnesium levels increase in fetal serum within 1 hour and AF within 3 hours after maternal IV administration.