Fetal blood velocity waveforms in uncomplicated pregnancy.

Umbilical artery velocity waves were measured in fetuses from 94 normal pregnant women. In all, 183 determinations were carried out from the 14th to the 40th week of gestation. A combination of pulsed echo and real-time scanning was used to obtain blood waveforms from the umbilical arteries. The umbilical artery velocity wave can be readily differentiated from other fetal signals by its pattern. The systolic peak of the velocity wave was divided by the end diastolic value, thereby giving an S/D ratio. The S/D ratio in normal pregnancy declined from 7.6 to 2.0 from 14 to 40 weeks. Analysis of these waveforms indicated that the placenta is an organ of low vascular resistance and that placental resistance to blood flow declines with advancing gestational age in normal pregnancy. The umbilical artery S/D ratio provides a new and non-invasive marker of fetoplacental blood flow resistance.