Evaluation of complex cardiovascular hemodynamics requires information concerning heart rate, peripheral resistance, blood pressure, and cardiac contractility. Cardiovascular assessment is usually based on Doppler ultrasonographic and cardiotocographic evidence. Fetal circulation must be studied noninvasively. Noninvasive measurement of pulsatile diameter changes (pulse waveform) in the fetal descending aorta (DA) and inferior vena cava (IVC) during fetal development using an ultrasonic phase-locked echo tracking system at a sampling frequency of 5000 Hz provides important physiologic information. Simultaneous recording of pressure and diameter pulse waveforms in humans has shown their concordance. These waveforms may be used to represent the direction and relative magnitude of changes in arterial and venous pressure, and thus make it possible to monitor variation in blood pressure in cardiac afterload occurring through fetal development.