Antenatal fetal blood flow in the descending aorta and in the umbilical vein and their ratio in normal pregnancy.

One hundred and sixty sequential measurements of umbilical vein blood flow (UVBF) and descending aorta blood flow (DABF) in normal fetuses were performed in utero by the duplex real-time ultrasound and pulsed Doppler technique. Throughout pregnancy from 26 to 41 weeks the blood flow velocity of the umbilical vein (UV) and the descending aorta (DA) remained relatively constant, while the diameter of UV and DA, UVBF and DABF increased with advancing gestational age. Moreover, the UVBF-to-DABF ratio was nearly constant, with a mean value of 64% from 26 weeks onwards. The DABF-to-abdominal area (DABF-to-AA) ratio was constant throughout pregnancy, with a mean value of 7.0 ml/min/cm2. We call these two constants the umbilical-aortic index and descending-aortic index. These relationships may be of help in the diagnosis of intrauterine growth retardation and other fetal disorders.