Fetal MCA systolic/diastolic ratio

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. 

Ultrasound - obstetric
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Article information

rID: 25519
System: Obstetrics
Section: Approach
Tag: refs, cases
Synonyms or Alternate Spellings:
  • Fetal MCA systolic/diastolic (S/D) ratio
  • Fetal MCA S/D ratio
  • Fetal MCA S/D ratios

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