Calculate MCA peak systolic velocity multiples of median (MoM) for fetal anemia detection using the Mari regression equation with interpretation guide.
Middle cerebral artery peak systolic velocity (MCA-PSV) Doppler has revolutionized the non-invasive detection of fetal anemia, largely replacing amniocentesis for bilirubin measurement (delta OD450) as the primary surveillance tool in Rh and other red cell alloimmunization. Anemic fetuses develop a hyperdynamic circulation — reduced blood viscosity and increased cardiac output raise velocities in major vessels, particularly the MCA. This calculator converts raw MCA-PSV measurements into Multiples of Median (MoM) using the Mari regression equation published in the landmark 2000 New England Journal of Medicine study.
The critical threshold of 1.5 MoM identifies moderate-to-severe fetal anemia with approximately 100% sensitivity and 88% specificity, with a positive predictive value of ~65-85% depending on the population studied. When MCA-PSV reaches or exceeds this threshold, cordocentesis (percutaneous umbilical blood sampling) is recommended to confirm fetal hemoglobin and, if confirmed, proceed with intrauterine transfusion (IUT). This non-invasive approach has dramatically reduced the number of invasive procedures needed in alloimmunized pregnancies.
This calculator also supports twin pregnancies for evaluation of Twin Anemia-Polycythemia Sequence (TAPS) in monochorionic twins, provides a post-transfusion reliability warning, and includes a complete reference table of median values and thresholds by gestational age.
MCA-PSV Doppler has become the standard of care for non-invasive fetal anemia screening in alloimmunized pregnancies. This calculator provides instant MoM calculation with the validated Mari regression, gestational age-specific thresholds, twin TAPS evaluation, and management guidance — essential for maternal-fetal medicine practitioners and sonographers. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain.
Median MCA-PSV = 10^(0.4437 + 0.0281 × GA weeks) cm/s (Mari et al. NEJM 2000). MoM = Measured PSV ÷ Median. Anemia threshold: MoM ≥ 1.5.
Result: Median = 38.1 cm/s, MoM = 1.44 — approaching threshold, repeat in 3-7 days
At 30 weeks, median MCA-PSV is ~38.1 cm/s. With a measured PSV of 55 cm/s, MoM = 55/38.1 = 1.44, which is moderately elevated but below the 1.5 MoM threshold. Close surveillance is warranted.
Before MCA-PSV Doppler, detection of fetal anemia relied on serial amniocentesis to measure amniotic fluid bilirubin (delta OD450 plotted on the Liley or Queenan curves). This invasive approach carried a 1-2% procedure-related complication rate per tap and the risk of further fetomaternal hemorrhage worsening the alloimmunization. The landmark Mari et al. NEJM 2000 study demonstrated that MCA-PSV ≥ 1.5 MoM detected all cases of moderate-severe anemia with a false-positive rate of only 12%, dramatically reducing the need for invasive testing.
When MCA-PSV confirms the need for intervention, cordocentesis is performed under ultrasound guidance, typically accessing the umbilical vein at its insertion into the placenta. Fetal hemoglobin is measured, and if anemia is confirmed (typically Hgb < 0.65 MoM for gestational age), packed red blood cells (irradiated, CMV-negative, O Rh-negative, antigen-matched) are slowly transfused to raise fetal Hgb to ~14-16 g/dL. IUT can be performed from approximately 18-20 weeks through 34-35 weeks, after which delivery is typically preferred.
While Rh alloimmunization remains the most common indication, MCA-PSV monitoring is now standard for any condition causing fetal anemia: Kell antibodies (which uniquely suppress erythropoiesis in addition to causing hemolysis), parvovirus B19 infection (transient aplastic crisis lasting 4-8 weeks), massive fetomaternal hemorrhage, and alpha-thalassemia major (Bart's hydrops). Each condition has nuances — Kell disease shows earlier and faster MCA-PSV rise, while parvovirus may resolve spontaneously as fetal immune response develops.
The most common cause worldwide is Rh (anti-D) alloimmunization, where maternal antibodies cross the placenta and destroy fetal red blood cells. Other causes include antibodies to Kell (K), c, and E antigens; parvovirus B19 infection (which directly suppresses fetal erythropoiesis); fetomaternal hemorrhage; alpha-thalassemia; and twin anemia-polycythemia sequence (TAPS) in monochorionic twins.
The MCA should be imaged at the circle of Willis in an axial section of the fetal head. The Doppler gate is placed on the proximal third of the MCA near its origin from the internal carotid artery. The angle of insonation should be as close to 0° as possible (< 15°). The measurement should be taken during fetal quiescence (not during breathing movements or active state) and the highest PSV from 3-5 consistent waveforms is recorded.
In fetal anemia, decreased red cell mass leads to reduced blood viscosity and lower oxygen-carrying capacity. The fetus compensates with increased cardiac output and preferential blood flow redistribution to vital organs (brain-sparing). Both the reduced viscosity and increased cardiac output raise peak velocities in cerebral arteries, making MCA-PSV an excellent non-invasive surrogate for fetal hemoglobin level.
Reliability decreases after IUT. Adult donor red cells are smaller and more deformable than fetal red cells, altering flow dynamics. The false-negative rate for detecting re-anemia increases to 13-17% post-transfusion. Many centers use a lower threshold (1.3-1.5 MoM) for re-transfusion decisions and combine MCA-PSV with other Doppler indices (ductus venosus) and cardiotocography.
In monochorionic diamniotic (MCDA) twins, MCA-PSV is used to diagnose Twin Anemia-Polycythemia Sequence (TAPS), where one twin becomes anemic and the other polycythemic through small placental anastomoses. TAPS is diagnosed when the donor twin MCA-PSV is > 1.5 MoM and the recipient is < 1.0 MoM, with a delta MoM > 0.5 between the twins.
The Mari curve is validated from 16 to 35-37 weeks. After 36 weeks, there is increasing overlap between anemic and non-anemic MCA-PSV values, reducing specificity. Some centers use the curve cautiously up to 40 weeks, while others transition to biophysical profile and CTG monitoring in late pregnancy.