MCA-PSV Calculator (Middle Cerebral Artery)
The middle cerebral artery peak systolic velocity (MCA-PSV) is the standard noninvasive tool for detecting fetal anemia. This calculator converts a measured MCA-PSV into multiples of the median (MoM) for gestational age using the widely cited Mari regression equation, and provides a risk category for fetal anemia.
Fetal anemia, most commonly caused by Rh (D) alloimmunization, can lead to hydrops fetalis and fetal demise if not detected and treated. Before MCA Doppler became the standard, amniocentesis with spectrophotometry (delta OD450) was the primary screening tool, which carried procedural risks.
Mari et al. demonstrated in their landmark 2000 study (New England Journal of Medicine) that an MCA-PSV of 1.5 MoM or greater predicts moderate to severe fetal anemia with a sensitivity of approximately 100% for severe anemia and 88% for moderate anemia, with a false-positive rate of about 12%. This noninvasive approach has largely replaced amniocentesis for initial anemia screening.
The median MCA-PSV increases with gestational age as the fetal cardiac output rises. The regression equation (Mari 2005) accounts for this normal increase. When the fetus is anemic, the blood becomes less viscous and cardiac output increases to compensate, resulting in higher flow velocities in the MCA and other vessels.
Key MoM thresholds:
- Below 1.29 MoM: Low risk for significant anemia
- 1.29 to 1.5 MoM: Borderline; possible mild anemia; close follow-up needed
- 1.5 MoM or above: High risk for moderate to severe anemia; cordocentesis and possible intrauterine transfusion indicated
MCA-PSV measurement technique matters. The angle of insonation should be as close to zero degrees as possible, and the measurement must be taken from the proximal portion of the MCA near the Circle of Willis. Poor technique can lead to falsely elevated or reduced values.
This calculator is for educational reference. All clinical decisions should involve a maternal-fetal medicine specialist.