Forty-two preeclamptic patients between 36 and 41 weeks gestation were investigated for baseline fetal heart rate (FHR) variability 1 hour after the initiation of magnesium therapy, at midlabor, and at the time of delivery. With a standard regimen of intravenous magnesium sulfate therapy, the mean maternal serum magnesium levels at the three different sampling times were statistically different (f = 6.94, p less than 0.01 by one-way analysis of variance), documenting the cumulative effect of continuous intravenous magnesium administration. The majority of the fetuses (86%) that exhibited a decrease in long-term FHR variability were associated with a maternal serum magnesium level above the lower limit of the therapeutic range (4.8 mg/dl). Of all fetuses whose maternal serum magnesium levels were 4.8 mg/dl or greater at delivery, only 40% exhibited a decrease in FHR variability. A higher mean maternal magnesium level, a higher mean cord blood magnesium level, and a higher total dose of magnesium sulfate were observed in the group of fetuses showing a decrease in FHR variability than in the group showing no change in FHR variability. The good fetal outcome seen in both study populations suggests that the effect of magnesium sulfate on FHR variability is a transient, reversible phenomenon that should not be considered as a sign of fetal distress.

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