Longitudinal evaluation of uteroplacental and umbilical blood flow changes in normal early pregnancy.

The hemodynamic changes in the uteroplacental circulation and the umbilical artery were evaluated in normal pregnancy from 8 to 14 weeks' gestation. A 6-9-MHz broad-band transvaginal sonographic transducer combined with pulsed color Doppler was used to scan 37 healthy volunteer pregnant women at weekly intervals. Vascular impedance to blood flow in all examined vessels decreased significantly throughout the first gestational trimester. Resistance to flow was highest in the main uterine artery and decreased towards the spiral artery. When the flow velocity waveform patterns of the arteries under investigation were analyzed, specific changes were observed. The diastolic notch was present in the spiral artery flow velocity waveform in all cases until 10 weeks' gestation. From then, the diastolic notch disappeared progressively and was absent in 100% of cases at 13 weeks. The diastolic notch in the arcuate artery disappeared within 2 weeks of its disappearance in the spiral artery. We could not detect intervillous blood flow during the first 12 weeks. From then on, intervillous flow was observed in most pregnancies, reaching 100% at 14 weeks' gestation. These changes reflect the growth and development of the uteroplacental circulation. The absence of detectable intervillous flow during most of the first trimester confirms the concept that, during the first 3 months of gestation, blood flow to the intervillous space is inhibited by trophoblastic plugs.