MCA-PSV Doppler Calculator (Fetal Anemia)

About the MCA-PSV Doppler Calculator (Fetal Anemia)

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.

When This Page Helps

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.

How to Use the Inputs

1. Enter the gestational age in weeks (valid range: 16-40 weeks).
2. Enter the measured MCA peak systolic velocity and select the unit (cm/s or mm/s).
3. For twin pregnancies, enable twin mode and enter both co-twin PSV values for TAPS screening context.
4. Note if prior intrauterine transfusion has been performed (affects reliability).
5. Review MoM, threshold, and follow-up context.

Example Calculation

Tips & Best Practices

• Always use angle correction < 15° — PSV overestimation at larger angles leads to false positives.
• Measure during fetal quiescence — active fetal state raises PSV by up to 20%.
• Use the highest PSV from 3-5 consistent waveforms, not an average.
• Trending MCA-PSV over serial measurements is more informative than a single value.
• After IUT, many teams interpret the usual MoM thresholds more cautiously.
• Kell alloimmunization suppresses erythropoiesis — MCA-PSV may rise more rapidly than in Rh disease.

The Evolution of Fetal Anemia Detection

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.

Intrauterine Transfusion (IUT)

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.

Beyond Rh Disease

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.

Frequently Asked Questions

• What causes fetal anemia?

  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.

• How is the MCA Doppler performed correctly?

  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.

• Why does fetal anemia raise MCA velocity?

  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.

• Is MCA-PSV reliable after intrauterine transfusion?

  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.

• What is the role of MCA-PSV in twin pregnancies?

  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.

• At what gestational age can MCA-PSV be used?

  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.