A prospective longitudinal study determined the physiological patterns of blood flow velocity waveforms in normal and in small-for-gestational-age (SGA) fetuses. Using a Pulsed Doppler Duplex system, 125 normally grown fetuses and 35 SGA fetuses were studied longitudinally at between 27 and 42 weeks gestation. In normal fetuses the ratio between the E velocity (early passive ventricular filling) and the A velocity (active ventricular filling during atrial contraction) increased progressively during pregnancy in both transmitral and transtricuspid waveforms, approaching 1 at term. In SGA fetuses, the E/A ratios did not increase during pregnancy and the values obtained were significantly lower than in normal fetuses. The ratio between the transtricuspid and transmitral mean temporal velocities remained almost constant throughout pregnancy in the normal fetuses with transtricuspid velocity slightly exceeding the transmitral velocity. On the other hand, in SGA fetuses the ratio between the mean temporal velocities was inversed with the transmitral velocity progressively greater than the transtricuspid velocity suggesting the existence of intracardiac haemodynamic changes in these fetuses.