MCA PSV Calculator

What is the MCA PSV?

The Middle Cerebral Artery Peak Systolic Velocity (MCA PSV) is a Doppler ultrasound measurement used to non-invasively assess fetal anemia. In anemic fetuses, reduced blood viscosity (due to lower hemoglobin) leads to increased blood flow velocity, which can be detected by measuring the peak systolic velocity in the middle cerebral artery.

This technique, developed and validated by Dr. Giancarlo Mari and colleagues, has largely replaced invasive amniocentesis for the detection of moderate-to-severe fetal anemia. The MCA PSV is expressed as a Multiple of the Median (MoM) for the gestational age, allowing standardized interpretation across different gestational ages.

Mari Reference Curve Formula

The expected median MCA PSV for a given gestational age is calculated using the Mari regression equation:

Where GA is the gestational age in weeks. The MoM is then calculated as:

For twin pregnancies, the same formula is applied, but interpretation should account for the specific clinical context (e.g., twin-to-twin transfusion syndrome in monochorionic twins).

MoM Interpretation

MCA PSV MoM	Interpretation	Clinical Significance	Recommended Action
< 1.0	Normal	No evidence of fetal anemia	Routine surveillance per protocol
1.0 – 1.29	Mildly Elevated	Possible mild anemia	Closer monitoring, repeat in 1–2 weeks
1.29 – 1.5	Moderately Elevated	Moderate anemia risk	Close surveillance, consider cordocentesis
≥ 1.5	Significantly Elevated	High risk of moderate-severe anemia	Cordocentesis & consider intrauterine transfusion
≥ 1.55	Highly Elevated	Severe anemia likely (sensitivity ~100%)	Urgent evaluation for intrauterine transfusion

The 1.5 MoM threshold has a sensitivity of approximately 100% for detecting moderate-to-severe fetal anemia (hemoglobin deficit > 5 g/dL), with a false-positive rate of approximately 12% (Mari et al., NEJM 2000).

MCA PSV Reference Curve Diagram

Fetal Anemia & Rh Isoimmunization

Fetal anemia is a condition where the fetus has an abnormally low hemoglobin concentration. The most common cause is Rh isoimmunization (also called Rh disease or erythroblastosis fetalis), which occurs when an Rh-negative mother develops antibodies against the Rh-positive red blood cells of her fetus.

The mechanism involves:

1. An Rh-negative mother is exposed to Rh-positive fetal blood (during delivery, miscarriage, or invasive procedures)
2. The mother's immune system produces anti-D (Rh) antibodies
3. In subsequent Rh-positive pregnancies, these antibodies cross the placenta
4. The antibodies attack fetal red blood cells, causing hemolysis (breakdown)
5. This leads to fetal anemia, which can progress to hydrops fetalis (fluid accumulation) and fetal death if untreated

Other causes of fetal anemia include: Kell alloimmunization, parvovirus B19 infection, alpha-thalassemia, fetomaternal hemorrhage, and twin-to-twin transfusion syndrome.

Doppler Ultrasound Technique

Accurate MCA PSV measurement requires proper technique:

• Identify the Circle of Willis in an axial plane at the level of the sphenoid bone wings
• Select the proximal MCA (closest to the transducer, near the internal carotid artery bifurcation)
• Angle of insonation should be as close to 0 degrees as possible (ideal: < 15 degrees)
• Use pulsed-wave Doppler with the sample gate placed at the proximal third of the MCA
• Measure during fetal quiescence — fetal breathing or movement can alter the measurement
• Take the highest PSV from at least 3 consecutive uniform waveforms
• Do not apply angle correction — the original Mari data did not use angle correction

Expected PSV by Gestational Age

GA (weeks)	Median PSV (cm/s)	1.5 MoM (cm/s)	GA (weeks)	Median PSV (cm/s)	1.5 MoM (cm/s)
16	23.1	34.7	29	42.3	63.5
18	25.5	38.2	30	44.6	66.9
20	28.1	42.2	32	49.6	74.4
22	31.0	46.5	34	55.2	82.7
24	34.2	51.3	36	61.3	92.0
26	37.8	56.6	38	68.2	102.3
28	40.1	60.2	40	75.9	113.8

Worked Example

A singleton pregnancy at 28 weeks GA with measured MCA PSV of 40 cm/s:

This MoM of 1.00 is at the median line, which is normal. No evidence of significant fetal anemia. Routine surveillance per protocol is recommended.

The 1.5 MoM threshold at 28 weeks would be: 40.1 × 1.5 = 60.2 cm/s. A measured PSV above this value would raise concern for moderate-to-severe fetal anemia.

Frequently Asked Questions

What does MoM mean?

MoM stands for Multiple of the Median. It is the ratio of the measured value to the expected median value for a given gestational age. A MoM of 1.0 means the measurement is exactly at the median; a MoM of 1.5 means the measurement is 50% above the expected median.

How often should MCA PSV be monitored?

In Rh-alloimmunized pregnancies or other conditions at risk for fetal anemia, MCA PSV is typically measured every 1-2 weeks starting from 16-18 weeks gestation. Frequency may increase if values are rising or approaching the 1.5 MoM threshold.

Is MCA PSV accurate after intrauterine transfusion?

MCA PSV becomes less reliable after the first intrauterine transfusion because adult donor red blood cells have different viscosity properties than fetal cells. After transfusion, timing of subsequent transfusions is often guided by a combination of MCA PSV, estimated fetal blood volume calculations, and clinical judgment. The false-positive rate increases significantly after transfusion.

How is this different from umbilical artery Doppler?

Umbilical artery Doppler assesses placental resistance and is used to evaluate fetal growth restriction and placental insufficiency. MCA PSV specifically assesses the velocity of blood flow in the fetal brain artery, which increases with anemia due to decreased blood viscosity and increased cardiac output. They evaluate different conditions.

Does the formula apply to twin pregnancies?

The Mari formula can be applied to twin pregnancies, but interpretation must account for the specific clinical context. In monochorionic twins with twin-to-twin transfusion syndrome (TTTS), the donor twin may show elevated MCA PSV due to anemia, while the recipient twin may have normal or decreased values. MCA PSV discordance > 1.0 MoM between twins is a concerning finding.