Fetal MCA systolic/diastolic ratio

Last revised by Dr 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). 

Interpretation

Normal 

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. 

Abnormal

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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