Blood flow velocities of the umbilical arteries were measured by Doppler ultrasonography during variable decelerations of the fetal heart rate. The flow velocity waveforms, being normal between uterine contractions, showed either an unchanged flow velocity waveform with an exclusive fetal heart rate effect on end-diastolic velocities or a rapid change to absent and reverse diastolic flow during the decelerations, indicating an abrupt increase in placental resistance with a halt in placental perfusion. Computer-aided reconstruction of the fetal heart rate curve revealed the exact temporal relationship between the reduction of umbilical artery perfusion and deceleration of fetal heart rate. We showed that variable decelerations of fetal heart rate can be observed during only slightly changed umbilical perfusion or can be caused by a halt in placental perfusion, which does not necessarily mean an absence of any movement of the fetal blood column, but is a result of a systolic forward and diastolic reverse flow to the same extent.