Middle Cerebral Artery is the Best Artery for Detecting Fetal Anemia
The middle cerebral artery (MCA) peak systolic velocity measured by Doppler ultrasound should be used as the primary technique to detect fetal anemia, not the umbilical artery. 1
Why MCA is Superior
The Society for Maternal-Fetal Medicine explicitly recommends MCA peak systolic velocity (MCA-PSV) as the primary screening tool for fetal anemia across all causes, including red cell alloimmunization, parvovirus infection, twin-twin transfusion syndrome, and fetomaternal hemorrhage. 1 This recommendation is based on:
Sensitivity approaching 100% (95% CI: 0.86-1.0) for detecting moderate to severe fetal anemia with only a 12% false-positive rate when using a single MCA-PSV measurement. 1
Superior performance compared to amniocentesis, which is why amniotic fluid delta OD450 should not be used to diagnose fetal anemia. 1, 2
Universal applicability regardless of the underlying cause of anemia, making it the most versatile screening tool. 1
The Physiologic Basis
As fetal hemoglobin decreases, blood viscosity decreases and cardiac output increases to maintain oxygen delivery, resulting in increased blood flow velocity in the MCA. 1 This creates a strong inverse correlation between MCA-PSV and fetal hemoglobin concentration, particularly in moderate to severe anemia. 3, 4
Critical Technical Requirements
Proper technique is essential because incorrect measurements by inexperienced operators can cause more harm than good. 2 The measurement must be performed:
At the proximal MCA, 2 mm after its origin from the internal carotid artery (this location has the best reproducibility with intraclass correlation of 0.98-0.99). 1, 5
With the ultrasound beam as close to zero degrees angle as possible, parallel to the artery without angle correction. 1
During fetal quiescence (no breathing or movements) to avoid heart rate effects on velocity. 1
Repeated at least 3 times, using the highest MCA-PSV value for clinical decisions. 1
Diagnostic Threshold
MCA-PSV >1.5 multiples of the median (MoM) indicates significant risk for severe fetal anemia and warrants fetal blood sampling with preparation for intrauterine transfusion. 1, 6 This threshold has been validated across multiple studies and etiologies of fetal anemia. 7, 3
Why Not the Umbilical Artery?
The umbilical artery is not used for detecting fetal anemia because it does not demonstrate the same reliable velocity changes in response to decreasing hemoglobin levels. The MCA shows a predictable increase in peak systolic velocity with worsening anemia due to compensatory hemodynamic changes, while umbilical artery Doppler primarily reflects placental resistance and is used for different clinical indications (placental insufficiency, growth restriction). 1, 2
Common Pitfall
Approximately 10% false-positive rate exists even with proper technique, meaning some fetuses with elevated MCA-PSV may not be severely anemic. 6 However, this is still superior to alternative screening methods and justifies the recommendation for MCA-PSV as the primary diagnostic tool. 1