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Fetal bradycardia secondary to magnesium sulfate therapy for preterm labor. A case report.

BACKGROUND
Alterations in fetal heart rate variability and baseline may be seen with maternal administration of magnesium sulfate. In this case, a dose-related decrease in the baseline fetal heart rate was observed in association with magnesium sulfate administration.


CASE
A primiparous woman was given parenteral magnesium sulfate for preterm labor. After tocolytic administration, the fetal heart rate baseline decreased to 110 beats per minute (bpm) from its initial rate of 140 bpm, although good variability was maintained. Increasing the dosage was accompanied by a further drop in the baseline heart rate, to 100 bpm. Fetal echocardiography was normal except for sinus bradycardia. Stopping magnesium sulfate administration was accompanied by a return to the pretherapy baseline heart rate. The pregnancy progressed without any further complications, and a healthy infant was delivered three weeks later.


CONCLUSION
Maternal administration of magnesium sulfate may be associated with a profound decrease in the baseline fetal heart rate, resulting in fetal sinus bradycardia.