What are the problems and difficulties in exploring the Doppler of the fetoplacental circulation?

The frequency of fetal heart rate and fetal respiratory movements can significantly alter arterial waves.
The use of color and duplex Doppler ultrasound can optimize the angle of incidence in umbilical vessels and improve signal clarity.
The resistance and pulsatility indices show a gradual reduction as gestational age increases.
The clinical use of Doppler studies of the umbilical artery should reflect research results and, whenever possible, record signals in a free loop of the umbilical cord.