Umbilical artery velocity waves were measured in the fetuses from 130 pregnant women. One hundred eighty-five determinations were carried out from the fourteenth to the fortieth weeks of pregnancy. Detection of waveforms was carried out on an Angioscan Doppler spectrum analyzer. The umbilical artery velocity waves can be differentiated from other fetal signals by recognition of the pattern. The systolic peak of the velocity wave was divided by the end-diastolic value, thereby giving an S/D ratio. The use of a ratio overcomes the obstacle of not knowing the angle between the incident beam and the direction of motion. The umbilical velocity wave S/D ratio in normal pregnancies declines from 2.8 to 2.2 from 25 to 41 weeks. In pregnancies which result in a small-for-gestational age (SGA) fetus, the ratio is significantly higher, showing an average level of 3.8 at 29 weeks and declining to 3.0 at 40 weeks. Abnormal umbilical velocity wave values are seen in an SGA fetus, unexplained fetal death, poorly controlled diabetes mellitus, and a twin transfusion syndrome. Application of this technique has the potential of being an important aid in prenatal care.