The MCA calculator converts a middle cerebral artery peak systolic velocity (MCA-PSV) reading from a fetal Doppler ultrasound into a Multiple of the Median (MoM) value. Sonographers and maternal-fetal medicine clinicians use this MCA-PSV calculation to screen pregnancies for fetal anemia without needing an invasive procedure. Enter the gestational age and the measured velocity, and the calculator returns a MoM figure along with an interpretation of anemia risk.
How to use this MCA calculator
- Enter the gestational age in the two fields provided: weeks and days. The calculator is validated for a gestational age range of 14 to 40 weeks.
- Enter the measured MCA peak systolic velocity from the Doppler ultrasound reading.
- Select the correct unit for that velocity from the dropdown: inches per minute (in/min), millimeters per minute (mm/min), or centimeters per second (cm/s).
- Click “Calculate MoM” to run the calculation.
- Review the output, which shows the Multiple of the Median (MoM) value along with an interpretation label indicating whether the result falls below or above the 1.5 MoM threshold associated with moderate-to-severe fetal anemia risk.
- Optionally, use the “Save This Assessment” section to keep a record of the result, with image and PDF export available, and view or clear prior entries in the “Saved Assessments” list.
What this MCA calculator does
The middle cerebral artery is one of the main vessels supplying blood to the fetal brain, and it’s clearly visible on Doppler ultrasound, which is what makes MCA-PSV screening practical in a routine scan. When a fetus becomes anemic, blood viscosity drops, and the fetal heart compensates by pumping faster to keep oxygen delivery to the brain steady. That compensation shows up as an elevated peak systolic velocity in the MCA. This MCA calculator takes the velocity you measured and compares it against the expected median velocity for that gestational age, producing a standardized MoM value. A MoM near 1.0 means the reading is close to the population median for that gestational age; a MoM well above 1.0 signals a velocity higher than expected, which correlates with anemia.
This kind of screening is used mainly in pregnancies with an elevated risk of fetal anemia, such as those involving Rh or red-cell alloimmunization, parvovirus B19 infection, or severe fetal growth restriction. It gives clinicians a non-invasive way to decide whether further workup, monitoring, or intervention is warranted, instead of going straight to an invasive test.
The MCA-PSV formula and MoM calculation
The calculator is built on the regression formula published by Mari and colleagues in 2000, which models the natural log of the expected peak systolic velocity as a function of gestational age:
ln(PSV) = 2.31 + 0.04643 × GA
where GA is gestational age in weeks. The calculator first converts your entered velocity into the expected unit, computes the predicted median PSV for the entered gestational age from this equation, and then divides your measured PSV by that predicted median to produce the MoM value shown in the result.
MoM interpretation reference
| MoM value |
Interpretation |
| Less than 1.5 |
Reassuring against moderate/severe fetal anemia |
| 1.5 or greater |
Threshold associated with moderate-to-severe fetal anemia, warranting clinical evaluation |
Why unit selection matters
Doppler machines and reporting systems don’t all record peak systolic velocity in the same unit, so this MCA calculator lets you pick between in/min, mm/min, and cm/s before running the calculation. Choosing the wrong unit will throw off the MoM result substantially, since the formula compares your value against a predicted velocity, so it’s worth double-checking the unit shown on the ultrasound report before you enter a number.
Working outside the validated range
The underlying formula was validated for gestational ages between 14 and 40 weeks. Entering a gestational age outside that window means the predicted median PSV, and therefore the MoM output, is extrapolated beyond the data the formula was built on, so results should be interpreted with extra caution at the edges of that range.
This MCA calculator is intended as a screening aid to help interpret a Doppler measurement, not as a substitute for clinical judgment. Any MoM result, especially one at or above the 1.5 threshold, should be reviewed by a qualified maternal-fetal medicine specialist alongside the full clinical picture before any decisions about further testing or management are made.
Frequently asked questions
What does MCA stand for in this calculator?
MCA stands for middle cerebral artery, one of the main blood vessels supplying the fetal brain. This calculator specifically works with the MCA peak systolic velocity (MCA-PSV) measurement taken by Doppler ultrasound.
What is a normal MCA-PSV MoM result?
A MoM below 1.5 is considered reassuring against moderate to severe fetal anemia. A MoM of 1.5 or higher is the threshold associated with an increased risk of moderate-to-severe fetal anemia and typically prompts further clinical evaluation.
Why does the MCA calculator ask for gestational age in both weeks and days?
The expected median peak systolic velocity changes continuously as pregnancy progresses, so entering the precise gestational age, down to the day, gives a more accurate predicted value to compare your measurement against, rather than rounding to the nearest week.
Can I use this MCA calculator for any gestational age?
The formula behind this calculator is validated for gestational ages between 14 and 40 weeks. Results calculated outside that range are extrapolated and should be interpreted cautiously alongside other clinical findings.
