Fetal central blood circulation in the third trimester of normal pregnancy--a longitudinal study. II. Aortic blood velocity waveform.

Waveform of the maximum blood velocity recorded from the fetal descending thoracic and abdominal aorta was analyzed in a longitudinal study on 21 normal pregnancies from the 27th gestational week till term. Measurements of blood velocity were performed with combined real-time linear array and 2 MHz pulsed Doppler ultrasound technique every second week. The rising (RS) and the descending (DS) slopes did not change with gestational age in the thoracic descending aorta. The peak maximum velocity, the pulsatility index (PI) and the acceleration time percentage were constant during the last trimester of pregnancy, in both the thoracic and the abdominal aorta. In the former, the mean RS was 29.9 (S.D. 4.9), mean DS 5.3 (S.D. 0.9), mean peak velocity 115.6 cm X s-1 (S.D. 19.0), mean PI 1.96 (S.P. 0.31) and the mean acceleration time percentage 19.2% (S.D. 2.2). In the abdominal aorta, the mean RS was 25.7 (S.D. 5.6), mean DS 4.5 (S.D. 0.9), mean peak velocity 99.7 cm X s-1 (S.D. 18.8), mean PI 1.68 (S.D. 0.28) and the mean acceleration time percentage 19.1% (S.D. 2.2). The PI and the DS, considered to reflect mainly peripheral vascular resistance, were mutually related (r = 0.72). The acceleration time percentage, RS and PI did not show any relation to fetal heart rate or gestational age which justifies the use of gestational age-independent reference values for those parameters during the last 3 months of gestation, at least within the normal fetal heart rate range.