Umbilical arterial pulsatility index

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

Practical points

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.

See also

• pulsatility index (general) 
• fetal middle cerebral artery pulsatility index
• automatic online fetal umbilical artery Doppler indices calculator from www.perinatology.com