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.