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Inferior vena cava (IVC) flow velocity waveforms represent both right atrial function and the blood flow pattern within the venous tree of the lower fetal body. Flow velocity waveforms obtained close to the venous entrance into the right atrium reflect mostly right atrial activity. Therefore the information obtained by studying blood flow patterns within the IVC includes (1) whether there is normal atrial filling; (2) detection of a fetal arrhythmia and the resulting blood flow pattern; and (3) estimation of hemodynamic disturbances in severely compromised fetuses. In this chapter we describe the normal pattern of IVC blood flow and the various components of the IVC flow velocity waveforms. Typical alterations of the IVC flow velocity waveforms in hydropic fetuses, fetuses with arrhythmias, and severely compromised fetuses are discussed. Finally, special attention is devoted to the significance of umbilical vein pulsations. [ABSTRACT FROM AUTHOR]