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To study the diameter pulse waveforms (DPWs) recorded noninvasively from the fetal inferior vena cava (IVC) in human fetuses.We studied 90 normal fetuses (20 to 40 weeks), ten fetuses with abnormalities of cardiac structure, and seven fetuses with arrhythmia. A paired ultrasonic phase-locked echo tracking system was used to follow the movement of diametrically opposite points of the IVC.The four component (A, X, V, Y) waves of the DPW were identified. In the normal group, there was an increase in the depth of × and Y nadirs. The abnormal group was divided into two subgroups. In 12 fetuses (five pulmonary stenosis, seven arrhythmia) there was a high pulsatile pattern with deep nadir from the A peak to × trough so that the pulsatility of the waveform appeared increased. The cause of the high pulsatility was due to a marked change of intraatrial pressure. In five fetuses with tricuspid regurgitation, this change was shallow and the pulsatility appeared reduced. Clinical outcome was significantly worse in the low pulsatile subgroup.The low pulsatility waveform may indicate depressed myocardial function. Measuring the DPW provides a simple method for obtaining important information about fetal cardiac performance.