Ultrasound examination of blood flow. I. Flow in the fetal aorta.

The descending part of the fetal aorta was examined in a total of 101 healthy pregnant women on 146 occasions between the 28th and 41st weeks of pregnancy. A Kranzbühler 8130 Duplex Doppler apparatus was used throughout. It was found that the blood flow compared to the estimated fetal weight was near to the same value (180-200 ml/min/kg) during the third trimester. The absolute volume of blood flow--not related to fetal body weight--increased about three times from the 28th week to term (200-600 ml/min). The diameter of the fetal aorta increased from 3-4 mm to 7-8 mm during the 3rd trimester. The velocity of blood flow fluctuated between wide limits (20-40 cm/sec), but the average was 28-32 cm/sec. The A/B ratios (A = systolic peak, B = end diastole), the Resistance Indexes (A-B/A), the Pulsatility Indexes (A-B/mean frequency) were nearly the same during the third trimester.