Effect of tocolytic agents on fetal umbilical velocimetry.

In this prospective study, we sought to examine the changes in umbilical vascular resistance induced by tocolytic therapy. Umbilical artery velocimetry was performed in 46 patients with preterm labor before tocolysis and at 1 hour and 24 hours after tocolysis was initiated. Raw systolic/diastolic ratios were corrected for concomitant changes in fetal heart rate. Thirty patients received subcutaneous terbutaline and 16 were treated with intravenous magnesium. Systolic/diastolic ratios decreased in patients treated with terbutaline. This decline persisted after correction for increases in fetal heart rate. No significant changes in systolic/diastolic ratios were seen in patients treated with magnesium. We conclude that terbutaline may affect umbilical vascular resistance. Possible clinical implications of these findings are discussed.