Abstract
The pulsatility index (PI) is now the most widely used index for assessing uterine artery Doppler waveform patterns. So it involves the portion below the curve, the PI best explains the shape of the velocity waveform. This review aimed to clarify the technical particularities and difficulties and to demonstrate also the factors that affect the Doppler flow velocity of the umbilical artery (UA). It was demonstrated that there are certain technical methods showing particularities and difficulties during intrapartum Doppler examination. The greatest important complications associated with the examination techniques include changes in circulatory parameters and fetal during uterus contractions, also the movements of maternal respiration could make it difficult to record Doppler signals, and reduced amniotic fluid volume difficulties of the Doppler examination of fetal vessels, particularly after the rupture of membranes. Additionally, some factors affect Doppler flow velocity waveforms of the umbilical artery (UA) such as gestational age, fetal heart rate, fetal respiration movement, measurement's site, UA radius, the experience of the users, impedance to pulsatile flow propagation, increasing of downstream resistance, and angle of the fetal Doppler insonation.