Screening for Fetal Anemia: Middle Cerebral Artery
The middle cerebral artery (MCA) is the vessel used to screen for fetal anemia, not the umbilical artery. 1, 2
Why MCA and Not Umbilical Artery
The MCA peak systolic velocity (MCA-PSV) is the primary screening method recommended by the Society for Maternal-Fetal Medicine for detecting fetal anemia of any etiology, including red cell alloimmunization, parvovirus infection, twin-twin transfusion syndrome, and fetomaternal hemorrhage. 1, 2
Umbilical artery Doppler does not reliably reflect fetal anemia because velocity changes in this vessel are not consistently related to hemoglobin levels; instead, umbilical artery Doppler primarily assesses placental resistance and is used for conditions such as placental insufficiency and fetal growth restriction. 2
Diagnostic Performance of MCA-PSV
MCA-PSV detects moderate to severe fetal anemia with nearly 100% sensitivity (95% CI: 0.86-1.0) and a false-positive rate of approximately 12%. 1, 3
A threshold of MCA-PSV >1.5 multiples of the median (MoM) signals significant risk of severe fetal anemia and warrants fetal blood sampling with preparation for possible intrauterine transfusion. 1, 2
MCA-PSV outperforms amniocentesis for anemia diagnosis; amniotic fluid delta OD450 should not be used for this purpose. 2, 4
Physiologic Basis
As fetal hemoglobin decreases, MCA-PSV increases due to compensatory hyperdynamic circulation, allowing determination of hemoglobin values with good approximation in moderate to severe anemia. 1
The technique is universally applicable regardless of the underlying cause of anemia, providing a versatile screening tool across multiple etiologies. 1, 2, 4
Proper Measurement Technique
Location and visualization:
- Measure at the proximal MCA, approximately 2 mm distal to its origin from the internal carotid artery, which yields excellent reproducibility (intraclass correlation 0.98-0.99). 2
- Obtain an axial section of the fetal head at the level of the sphenoid bones and image the circle of Willis with color Doppler. 1
- The entire length of the MCA should be visualized, occupying more than 50% of the image after zooming. 1
Technical parameters:
- Align the ultrasound beam as close to 0° as possible, parallel to the artery for its entire length, ideally without angle correction. 1, 2
- Acquire measurements during fetal quiescence (no breathing or movements) to minimize heart-rate-related velocity variation. 1, 2
- Perform at least three consecutive measurements and record the highest MCA-PSV value for clinical decision-making. 1, 2
Clinical Application
The procedure takes approximately 5-10 minutes when performed by trained operators. 1
Operators must be properly trained in the technique, as incorrect use by inexperienced personnel may cause more harm than good; patients at risk should be referred to centers with trained sonographers. 4
Serial measurements (trends) may decrease the false-positive rate to less than 5%, compared to single measurements. 1