OBJECTIVE To assess ductus arteriosus velocity waveforms in the appropriate and small-for-gestational-age (SGA) fetuses. METHODS Ductus arteriosus flow velocity waveforms were obtained in 14 normal fetuses in a longitudinal study and in 139 normal fetuses in a cross-sectional study with pulsed and continuous Doppler ultrasonography. Ductus velocity waveforms were also determined in 25 SGA fetuses whose gestational age ranged between 20 and 34 weeks (mean +/- SD: 30 +/- 4.1 weeks). RESULTS Mathematical modeling demonstrated that the peak systolic velocity, lowest diastolic velocity, and mean velocity of the ductus arteriosus increased with advancing gestation in both the longitudinal and cross-sectional study, but considerable individual variability was seen in the longitudinal study. The pulsatility index did not change significantly. The ductal parameters were in the normal range in all the SGA fetuses. CONCLUSIONS The blood velocity of the ductus arteriosus increase with advancing gestation. Our results suggest that Doppler ultrasonographic study of the fetal ductus arteriosus, unlike Doppler study of the umbilical artery or middle cerebral artery, is not useful as an indicator of adverse fetal outcome.