Fetal inferior vena cava (IVC) flow velocity waveform recording was attempted at the entrance into the right atrium in 60 women at 19-22 weeks, 27-30 weeks and 36-39 weeks of gestation. Technically acceptable waveforms were collected in 48 women. A significant increase in time-averaged velocity and a significant decrease in percent reverse flow with advancing gestational age was established. A large standard deviation was observed for various IVC waveform parameters. From preliminary postmortem data it appeared that the inferior venous entrance into the right atrium represents a funnel-like structure composed of the inlet of the IVC, hepatic veins and ductus venosus. It is suggested that waveform recording at the scanning level employed in the present study provides information on gestational age-related changes in venous return to the right atrium rather than changes in the IVC itself. It is proposed that information on IVC flow velocity waveforms should be obtained more distal to the venous entrance into the right atrium.