The role of cord blood viscosity in determining the umbilical artery Doppler flow velocity waveform (FVW) was investigated in 22 normal pregnancies and 29 complicated pregnancies. FVWs were quantified by calculating the pulsatility index (PI). There was a significant correlation between an abnormal PI (more than 2 SD from the mean) and fetal growth retardation (less than 5th birthweight centile), cesarean section for fetal distress, and raised cord blood hematocrit. However, there was no relationship between whole blood or plasma viscosity measurements and the umbilical artery PI.