Flow velocity wave forms in the human fetal ductus arteriosus during the normal second half of pregnancy.

Maximum flow velocity wave forms in the ductus arteriosus were studied longitudinally in 40 fetuses from 18 to 38 wk of gestation at 3- to 4-wk intervals. Median maternal age was 29 y (range 18-41 y), median parity was 1 (range 0-6). All flow velocity wave forms were obtained using a mechanical sector scanner with a continuous Doppler system (carrier frequency 3.5 MHz). The flow velocity parameters studied were peak-systolic, peak-diastolic, and time-averaged velocity and acceleration time. There was considerable variability in measurements between fetuses at any particular gestational age. An increase of ductal flow parameters with advancing gestational age was found. They showed no correlation with heart rate. These results suggest that the normal second half of pregnancy is characterized by a reduction in right ventricular afterload. This may be a result of decreased placental vascular resistance or increased pulmonary vascular flow. Acceleration time in ductal flow velocity wave forms was significantly higher than that observed in the pulmonary artery and ascending aorta, suggesting a lower afterload in the ductus arteriosus compared with the other two outflow tract vessels.