Umbilical arterial pulsatility index (UA-PI) is a parameter used in umbilical arterial (UA) Doppler assessment. It is calculated by subtracting the end-diastolic velocity (EDV) from the peak systolic velocity (PSV) and then dividing by the time-averaged (mean) velocity (TAV):
- PI = (PSV - EDV) / TAV
Reference ranges depend on the stage of gestation. In normal pregnancies, umbilical arterial PI starts at higher values and then decreases progressively towards term. This indicates that normal in placental development, there is a progressive decrease in the impedance of the umbilical vascular bed to blood flow.
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