Umbilical arterial pulsatility index

Last revised by Henry Knipe on 16 Feb 2024

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 pulsatility indexes start at higher values and then decrease progressively towards term. This indicates normal placental development, where there is a progressive decrease in the impedance of the umbilical vascular bed to blood flow.

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