BACKGROUND Blood flow is calculated from mean velocity across the vessel and its cross-sectional area and is related to the fetal growth. OBJECTIVE To investigate the relationship between diameter pulse waveform (DPW) and flow velocity waveform (FVW) in the fetal descending aorta during fetal development. METHODS Doppler ultrasound and a phase-locked loop echo tracking system were used to measure the FVW and DPW in the fetal descending aorta, respectively. METHODS We studied 137 normal-growth fetuses (normal group, 20-40 weeks) and 51 fetuses with high umbilical artery pulsatility index (umbilical placental insufficiency, UPI group, 26-40 weeks). METHODS We measured the systolic (Sd), diastolic (Dd) diameters, time diameter integral (TDI) and time velocity integral (TVI) and then calculated the TVI x TDI and TVI to TDI ratio. RESULTS Normal fetal growth was associated with an increase in Sd, Dd, pulse amplitude, TVI, TDI and TVI x TDI. The FVW began to resemble the DPW with decreasing downstream resistance produced by growth of the placenta. The TVI was increased relative to the TDI. The differences in Sd, Dd, TDI and TVI x TDI between the normal and UPI groups were not significant. The TVI was decreased relative to the TDI. There was a decrease in the TVI as a ratio of the TDI. The Dd per unit fetal weight was high in the compromised fetuses. Fetal outcome was examined in relation to the TVI to TDI ratio. Those with a low ratio (below 10th centile) exhibited significantly more adverse indices of fetal outcome. CONCLUSIONS In fetal compromise there is an increase in diastolic pressure in association with high placental resistance, which causes a major increase in afterload. The efficient circulation associated with fetal growth might be represented by an increase in the ratio of the TVI to the TDI (an index of efficient circulation) when these waveform shapes resemble each other.