Doppler blood flow waveforms in the descending thoracic aorta of the human fetus

A linear‐array pulsed Doppler duplex has been used to study third‐trimester Doppler flow velocity waveforms (FVW) in the descending thoracic aorta of 98 normal and 20 severely growth‐retarded fetuses. Waveforms have been normalized (frequency index profile; FIP) to remove the effect of the angle of insonation and so allow comparison at different gestational ages and with complicated pregnancies. There is an increase in end‐diastolic velocities with advancing gestational age indicating a reduction in placental resistance. The growth‐retarded fetuses showed a significant reduction in end‐diastolic velocities and an increase in the pulsatility index (normal PI: mean 1.83, SD 0.22; growth‐retarded PI: mean 2.78, SD 0.60). This reduction of diastolic blood flow may indicate reduced placental vascularity and predict impending fetal hypoxia.