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Fetal middle cerebral artery pulsatility index

Last revised by Assoc Prof Craig Hacking on 30 May 2020

The fetal middle cerebral artery (MCA) pulsatility index (PI) is a key parameter used in fetal middle cerebral arterial 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

The fetal MCA PI normally has a high value. The mean value (normal reference range) slowly decreases through gestation from around 28 weeks onwards. A low PI reflects the redistribution of cardiac output to the brain due to the fetal head sparing theory.

See also

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Cases and figures

  • Case 1: 3rd trimester normal MCA Doppler
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