Quantitative blood flow in the fetal descending aorta and in the umbilical vein in normal pregnancies. Longitudinal and cross-sectional studies

Abstract

Quantitative blood flow in the fetal descending thoracic aorta and in the umbilical vein was measured both in a longitudinal study where each of 13 fetuses was examined three times and in a cross-sectional study where 45 fetuses were examined once only in the last trimester of normal pregnancy. The waveform of the velocity curves from fetal descending aorta was examined and the Pulsatility Index, the A/B ratio and the least diastolic velocity were calculated. The longitudinal study showed that all these parameters and the fraction of the flow in the fetal descending aorta, which was distributed to the placental circulation, were constant during the last trimester. The problems of longitudinal inference from cross-sectional studies are discussed. For all 58 fetuses the flow in the descending thoracic aorta was 234 ml/min/kg and in the umbilical vein 127 ml/min/kg. The Pulsatility Index was 1.83, the A/B ratio 4.75, the least diastolic velocity 9.9 cm/s. The fraction of blood flow in the descending aorta distributed to the placental circulation was 55%.

Key words:

• A/B ratio
• Doppler flow
• Pulsatility Index