Fetal MCA systolic/diastolic ratio

Last revised by Avni K P Skandhan on 24 Mar 2016

Fetal MCA systolic/diastolic (S/D) ratio is an important parameter in fetal middle cerebral arterial Doppler assessment. It is a useful predictor of fetal distress and intrauterine growth restriction (IUGR). 



During pregnancy the middle cerebral (and other intracranial) arteries demonstrate high resistance waveforms, i.e. high systolic velocity and low/absent diastolic velocity.

The normal value of fetal MCA S/D ratio will vary by institution. A normal fetal MCA S/D ratio should always be higher than the umbilical arterial S/D ratio. The fetal MCA S/D ratio value will decrease as the pregnancy progresses. 


A reduced S/D ratio is abnormal and implies an increased diastolic flow in MCA. Loss of high resistance waveform is also an indicator of fetal distress or IUGR. 

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