MCA Calculator (middle cerebral artery PSV in MoM for fetal anaemia)

Understanding the MCA-PSV MoM calculator — concept and principle

This calculator converts the middle cerebral artery peak systolic velocity (MCA-PSV) measured by fetal Doppler ultrasound into multiples of the median (MoM) for the week of pregnancy. Fetal medicine teams use it to screen for fetal anaemia in pregnancies at risk, such as red-cell alloimmunisation or parvovirus infection.

Faster flow in fetal anaemia: with the default inputs the result is 1.37 MoM (below 1.5 MoM).

How it is calculated

Median PSV (cm/s) = e^(2.31 + 0.046 × GA)

MoM = measured PSV ÷ median PSV

1.5 MoM value = 1.5 × median

GA is gestational age in weeks, with days divided by 7.

The principle

When a fetus is anaemic its blood is thinner (lower viscosity) and the heart pumps more to deliver oxygen, so blood moves faster through the arteries. The middle cerebral artery is easy to measure at a near-zero angle, which gives accurate velocities. Mari and colleagues (2000) described how the median PSV rises with gestational age and showed that values of 1.5 MoM or more detected moderate and severe anaemia well without needing blood sampling.

Worked example

Default inputs: 28 weeks 0 days, PSV 50 cm/s.

Median = e^(2.31 + 0.046 × 28) = e^3.598 = 36.5 cm/s.
MoM = 50 ÷ 36.52 = 1.37.
1.5 MoM value = 1.5 × 36.52 = 54.8 cm/s.
1.37 is Below 1.5 MoM.

Each point is the calculator re-run with only “MCA peak systolic velocity (cm/s)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

MCA peak systolic velocity (cm/s)	MCA-PSV in multiples of the median (MoM)
12.5	0.34
25	0.68
37.5	1.03
50	1.37
62.5	1.71
75	2.05
100	2.74

How to read the result

Where does your result fall?

Values below 1.5 MoM are reassuring in this screening context, and scans are usually repeated at intervals in at-risk pregnancies. A value of 1.5 MoM or more suggests moderate or severe anaemia and usually leads to referral for further testing and possibly an intrauterine transfusion. False positives rise after about 35 weeks.

Common mistakes

Measuring with an angle of insonation well above 0°, which underestimates velocity.
Measuring far from the vessel's origin or while the fetus is moving or breathing.
Using the wrong gestational age.

Keep in mind: This is a calculation aid for scan results, not a diagnosis. Decisions belong to the fetal medicine team.

Frequently asked questions

What is a normal MCA-PSV MoM?

Values below 1.5 MoM are generally considered not to suggest moderate or severe anaemia.

Why is the MCA used?

It is easy to find and can be measured at a near-zero angle, giving reliable velocities.

Can the test be repeated?

Yes. In at-risk pregnancies it is often repeated every one to two weeks.

Sources: Mari G, Deter RL, Carpenter RL, et al., "Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization", N Engl J Med 2000;342:9–14 · Society for Maternal-Fetal Medicine, Clinical Guideline #8, "The fetus at risk for anemia: diagnosis and management", Am J Obstet Gynecol 2015;212:697–710

Checked 9 Nov 2026. Mari G, Deter RL, Carpenter RL, et al., “Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization”, N Engl J Med 2000;342:9–14 (median equation, standard value), and SMFM Clinical Guideline #8, “The fetus at risk for anemia”, Am J Obstet Gynecol 2015;212:697–710 (1.5 MoM threshold) — as cited by the Perinatology.com MCA-PSV calculator.

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