Clinical Note: MCA-PSV > 1.5 MoM is the evidence-based threshold for invasive testing (cordocentesis) or intrauterine transfusion in alloimmunized pregnancies.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
Primary Clinical Uses
Non-invasive screening for suspected fetal anemia (e.g., Rh/Kell alloimmunization)
Monitoring pregnancies at high risk for parvovirus B19 infection
Surveillance in cases of suspected twin-anemia polycythemia sequence (TAPS)
When NOT to Use
Predictive value drops precipitously after 35 weeks of gestation. Do NOT use MCA-PSV as the sole determinant for delivery timing in late preterm/term fetuses. False positive rates surge near term.
Anemia triggers a hyperdynamic circulatory state (increased cardiac output) to maintain tissue oxygenation.
The combination of thin blood and high output dramatically increases the peak systolic velocity (PSV) in the easily accessible Middle Cerebral Artery (MCA).
Standard Thresholds
< 1.5 MoM
Reassuring (Mild or no anemia)
≥ 1.5 MoM
High risk for Moderate-to-Severe Anemia
≥ 1.55 MoM
Often correlates strongly with hematocrit <30%
Section 3
Pearls/Pitfalls
Measurement Precision Rules
Angle of insonation MUST be exactly 0 degrees. Any angle correction artificially alters the velocity readout and invalidates the algorithm.
Measurement must be taken in the proximal third of the MCA, near its origin from the internal carotid.
Avoid applying pressure to the maternal abdomen (fetal head compression artificially elevates velocity).
Fetal breathing or overt movement will alter velocity; measure during a period of fetal apnea/quiescence.
If persistently ≥1.5 MoM and gestation < 35 weeks: Proceed to cordocentesis (Percutaneous Umbilical Blood Sampling - PUBS) for definitive hematocrit.
03
Prepare cross-matched, irradiated, O-negative packed red blood cells for immediate intrauterine transfusion (IUT) during PUBS if anemia is confirmed.
04
If ≥ 35 weeks: Consider expedited delivery rather than high-risk intrauterine transfusion.
Section 5
Evidence Appraisal
Landmark Trial
Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization.
Mari G et al. • N Engl J Med.. 2000;The pivotal study establishing 1.5 MoM as the golden threshold, demonstrating 100% sensitivity for moderate-to-severe fetal anemia.
Section 6
Literature
Dr. Giancarlo Mari
Pioneered by Giancarlo Mari and the Collaborative Group for Doppler Assessment of the Blood Velocity in Anemic Fetuses. It revolutionized maternal-fetal medicine by nearly eliminating the routine use of invasive, risk-heavy serial amniocenteses (Liley curves for delta OD450).
