Blood velocity waveforms were obtained by a combination of real-time B mode and pulsed Doppler (2 MHz) ultrasound from the fetal descending thoracic and abdominal aorta. In a prospective study 36 patients were studied. Throughout pregnancy 261 tracings of the thoracic aorta and 241 tracings of the abdominal aorta were evaluable for this study. Measurements were performed every 2nd week from a menstrual age of 18 weeks onwards till the 40th week. Reference values for the resistance index (RI), AB ratio and the pulsatility index (PI) were established in 27 patients with uncomplicated pregnancies. Abnormal waveforms were found in the small for gestational age (SGA) group, which consisted of 8 patients. A comparison was made between the 2 sample means of the PI in the abdominal and descending thoracic aorta in the group with appropriate for gestational (AGA) fetuses, as well as between the AGA and SGA groups. With regard to the 95% confidence interval significance is shown between the mean PI in the descending thoracic and abdominal aorta, as well as between the AGA and SGA groups. However, blood velocity measurements of the fetal aorta do not provide additional clinical information compared with measurements of the fetal aorta do not provide additional clinical information compared with measurements in the umbilical artery.