Umbilical artery waveform during bicycle exercise in normal pregnancy.

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Eighteen healthy pregnant women in the last half of gestation exercised to 85% of their predicted maximum heart rate on a bicycle ergometer. Pulsed Doppler flow studies before the exercise and during the recovery period were taken close to the umbilical artery's placental insertion site. The systolic-to-diastolic ratio (S/D) of the umbilical artery waveform did not change significantly during the period of observation. In three women, however, a significant increase in the S/D ratio occurred in the first few minutes post-exercise; this was due to a transient fetal bradycardia. In one woman, the S/D ratio remained high despite return of a normal fetal heart rate.