Time-velocity waveforms or sonograms obtained by pulsed Doppler ultrasound from the umbilical artery were analysed in a longitudinal study of 15 normal pregnancies. The sonograms from nine fetuses with intrauterine growth retardation were compared qualitatively and had quite different characteristics implying a high distal impedance to blood flow. Methods of analysis capable of differentiating between the normal and abnormal sonograms are described and discussed.