# MCA-PSV Calculator (Middle Cerebral Artery)

Calculate MCA-PSV multiples of the median (MoM) for fetal anemia screening. Uses the Mari regression for median peak systolic velocity by gestational age.

## What this calculates

The middle cerebral artery peak systolic velocity (MCA-PSV) is the standard noninvasive tool for detecting fetal anemia. This calculator converts a measured MCA-PSV into multiples of the median (MoM) for gestational age using the widely cited Mari regression equation, and provides a risk category for fetal anemia.

## Inputs

- **Gestational Age (weeks)** (weeks) — min 16, max 40 — Gestational age in completed weeks (16-40)
- **Additional Days** (days) — min 0, max 6 — Additional days beyond completed weeks
- **MCA Peak Systolic Velocity** (cm/s) — min 5, max 150 — Measured MCA-PSV from Doppler ultrasound in cm/s

## Outputs

- **Expected Median MCA-PSV** — Median MCA-PSV for this gestational age (Mari 2005)
- **MoM (Multiples of the Median)** — Measured PSV divided by the median for gestational age
- **Risk Category** — formatted as text — Fetal anemia risk based on MoM threshold
- **Clinical Recommendation** — formatted as text — Suggested follow-up based on MoM value

## Details

Fetal anemia, most commonly caused by Rh (D) alloimmunization, can lead to hydrops fetalis and fetal demise if not detected and treated. Before MCA Doppler became the standard, amniocentesis with spectrophotometry (delta OD450) was the primary screening tool, which carried procedural risks.

Mari et al. demonstrated in their landmark 2000 study (New England Journal of Medicine) that an MCA-PSV of 1.5 MoM or greater predicts moderate to severe fetal anemia with a sensitivity of approximately 100% for severe anemia and 88% for moderate anemia, with a false-positive rate of about 12%. This noninvasive approach has largely replaced amniocentesis for initial anemia screening.

The median MCA-PSV increases with gestational age as the fetal cardiac output rises. The regression equation (Mari 2005) accounts for this normal increase. When the fetus is anemic, the blood becomes less viscous and cardiac output increases to compensate, resulting in higher flow velocities in the MCA and other vessels.

**Key MoM thresholds:**

- **Below 1.29 MoM:** Low risk for significant anemia
- **1.29 to 1.5 MoM:** Borderline; possible mild anemia; close follow-up needed
- **1.5 MoM or above:** High risk for moderate to severe anemia; cordocentesis and possible intrauterine transfusion indicated

MCA-PSV measurement technique matters. The angle of insonation should be as close to zero degrees as possible, and the measurement must be taken from the proximal portion of the MCA near the Circle of Willis. Poor technique can lead to falsely elevated or reduced values.

This calculator is for educational reference. All clinical decisions should involve a maternal-fetal medicine specialist.

## Frequently Asked Questions

**Q: What does MoM mean in MCA-PSV measurement?**

A: MoM stands for multiples of the median. It is the measured MCA-PSV divided by the expected median value for that gestational age. A MoM of 1.0 means the value is exactly at the median. A MoM of 1.5 or higher is the threshold that predicts moderate to severe fetal anemia. Using MoM instead of raw velocity allows comparison across different gestational ages.

**Q: Why is MCA-PSV used to detect fetal anemia?**

A: When a fetus is anemic, its blood has lower viscosity (fewer red blood cells), and the heart compensates by increasing cardiac output. This combination results in higher blood flow velocities, particularly in the middle cerebral artery. The MCA is the vessel of choice because it is easy to image consistently, runs toward the ultrasound probe (favorable Doppler angle), and shows reliable velocity changes with anemia.

**Q: What causes fetal anemia?**

A: The most common cause is red cell alloimmunization, where maternal antibodies (most often anti-D/Rh) cross the placenta and destroy fetal red blood cells. Other causes include fetal parvovirus B19 infection, fetomaternal hemorrhage, twin-to-twin transfusion syndrome, and inherited hemoglobin disorders. Each of these conditions can be monitored with serial MCA-PSV measurements.

**Q: How often should MCA-PSV be measured in at-risk pregnancies?**

A: In Rh-alloimmunized pregnancies or other at-risk situations, MCA-PSV is typically measured every 1 to 2 weeks starting at 16 to 18 weeks gestation. The frequency may increase if values are trending upward or approaching 1.5 MoM. After an intrauterine transfusion, MCA-PSV becomes less reliable for 1 to 2 weeks and alternative monitoring may be needed.

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Source: https://vastcalc.com/calculators/health/mca
Category: Health & Fitness
Last updated: 2026-04-08