Doppler ultrasound of the uteroplacental and fetoplacental circulations: Placental location, normal values and reference curves.

Aim: Doppler ultrasound assessment of blood flow velocity waveforms from the maternal and fetal sides of the placenta in normal pregnancies, taking into consideration placental location, in order to establish normal values and reference curves. Method: A cross sectional study of uneventful singleton pregnancies with gestational ages from 18 to 41 weeks and later on healthy newborns was performed by continuous-wave-Doppler ultrasound. Doppler flow signals from the uterine artery or the corresponding ascending branch and the umbilical arteries were analysed using the resistance index RI. The RI-values from the uterine circulation were classified according to the proximity of the placenta. Normal values and reference curves were established on the basis of centiles. Results: After consideration of exclusion criteria, 757 pregnancies remained for evaluation. The resistance index reference curves of the uterine circulation were dependent on placental location, with proximal RI-values being lower, but with nearly constant course during the second half of pregnancy. In contrast, the umbilical artery RI-values were higher but showed a continuous decrease. Conclusion: In the second half of pregnancy, down stream flow resistance of the uterine circulation is very low and nearly constant but depends on placental location. The fetoplacental circulation shows high flow resistances which decrease with increasing gestational age. These reference curves are comparable with others where placental location is taken into consideration.