Convert MCA peak systolic velocity to multiples of median (MoM) using Mari reference values and review the result in fetal-anemia screening context.
Middle cerebral artery peak systolic velocity (MCA-PSV) Doppler is widely used as a non-invasive screening tool for fetal anemia in Rh and other red-cell alloimmunization settings. Anemic fetuses develop a hyperdynamic circulation, and reduced blood viscosity plus higher cardiac output raise velocities in major vessels, particularly the MCA. This page converts raw MCA-PSV measurements into Multiples of Median (MoM) using Mari reference values.
The 1.5 MoM threshold is commonly used as a higher-risk screening cutoff for moderate-to-severe fetal anemia, but it is still a screening threshold rather than a diagnosis by itself. Interpretation usually depends on gestational age, clinical indication, ultrasound technique, and whether transfusion has already occurred.
This worksheet also includes a twin comparison view for TAPS screening context, a post-transfusion reliability caution, and a reference table of median values and 1.5 MoM thresholds by gestational age.
MCA-PSV is useful because it turns a raw Doppler measurement into a gestational-age-adjusted ratio that is easier to interpret during fetal-anemia surveillance. Seeing the MoM value beside the threshold and the gestational-age median helps reduce the chance of comparing a measurement to the wrong baseline.
The twin mode is useful for TAPS screening, while the post-transfusion warning helps flag situations where the usual threshold is less dependable.
Median MCA-PSV = e^(2.31 + 0.04643 × GA weeks) cm/s (Mari reference values). MoM = Measured PSV ÷ Median. The 1.5 MoM threshold is commonly used as a higher-risk screening cutoff for moderate-to-severe fetal anemia.
Result: Median = 40.6 cm/s, MoM = 1.36 — moderately elevated but below 1.5 MoM
At 30 weeks, the Mari reference median used on this page is about 40.6 cm/s. With a measured PSV of 55 cm/s, MoM = 55 / 40.6 = 1.36, which is above the median but still below the common 1.5 MoM screening threshold.
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 falls in a higher-risk screening range, specialist teams may use cordocentesis to measure fetal hemoglobin directly and to decide whether intrauterine transfusion is appropriate. The exact intervention threshold varies with gestational age, indication, and local practice, so this page keeps that step in discussion context rather than trying to reproduce a treatment pathway.
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.
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This worksheet converts a measured MCA-PSV to MoM using the selected gestational-age median, then compares that value with the commonly used 1.5 MoM screening threshold. It is intended to support review of measurements already obtained on ultrasound, not to direct fetal treatment or replace specialist interpretation.
Twin-mode values are shown as a screening context aid for TAPS-style comparisons, and post-transfusion wording is deliberately cautious because reliability can be lower after IUT.
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 a widely used non-invasive screening surrogate for fetal anemia risk.
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 rises after transfusion, so many centers interpret the usual MoM thresholds more cautiously and combine MCA-PSV with the broader surveillance picture.
In monochorionic diamniotic (MCDA) twins, MCA-PSV is used in TAPS screening, where one twin becomes relatively anemic and the other relatively polycythemic through small placental anastomoses. A donor twin MCA-PSV above 1.5 MoM, a recipient below 1.0 MoM, and a delta MoM above 0.5 are commonly cited screening features.
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.