Effect of intravenous magnesium sulfate on the biophysical profile of the healthy preterm fetus.

Objective: The null hypothesis is that intravenous magnesium sulfate does not affect the biophysical profile of the healthy preterm fetus.

Study Design: Thirty-one fetuses of 25 patients between the gestational ages of 24 and 35 weeks, median 31.4 and mean (+/- SD) 30.4 (+/- 2.9), who required tocolysis for uterine contractions were prospectively studied. After normal fetal biophysical assessment was documented, intravenous magnesium sulfate was started as a 4 or 6 gm loading dose and then infused at 2 to 3.5 gm/hr to achieve tocolysis. Blood was drawn for measurement of maternal serum magnesium levels immediately before intravenous magnesium sulfate was administered and at 2 and 12 hours after the loading dose. Biophysical profiles, consisting of a possible 12 points, were performed at the same time as blood was drawn. Serum magnesium levels were compared with one-way analysis of variance for repeated measures and biophysical profile scores with Friedman's test. Statistical significance was considered p < 0.05.

Results: Mean (+/- SD) serum magnesium levels were 1.7 (+/- 0.1) mg/dl before infusion, 4.3 (+/- 0.6) mg/dl at 2 hours, and 5.2 (+/- 0.7) mg/dl at 12 hours (p < 0.001). Six fetuses did not have a 12-hour biophysical profile; three were delivered for severe variable decelerations, two progressed in labor, and in one tocolysis was discontinued. The median biophysical profile score was 11 before intravenous magnesium sulfate, at 2 hours, and at 12 hours after the loading dose. The biophysical parameters present and the percentage of fetuses with each parameter were as follows: breathing (> 30 seconds), 88% (22/25) before magnesium sulfate, 84% (21/25) at 2 hours, and 92% (23/25) at 12 hours; nonstress test (reactive), 84% (21/25) before magnesium sulfate, 68% (17/25) at 2 hours, and 80% (20/25) at 12 hours; movement (normal), 100% (25/25) before magnesium sulfate, 100% (25/25) at 2 hours, and 96% (24/25) at 12 hours.

Conclusion: Intravenous magnesium sulfate did not significantly alter the biophysical profile in the 25 fetuses evaluated by three biophysical profiles in spite of the significant increase in maternal serum magnesium levels.