About the MCA (Middle Cerebral Artery) Calculator
This calculator computes the middle cerebral artery peak systolic velocity (MCA-PSV) reading used in obstetric ultrasound as a non-invasive screen for fetal anemia. Instead of relying on invasive amniocentesis or cordocentesis as a first step, sonographers measure the peak blood-flow velocity in the fetal middle cerebral artery with Doppler ultrasound. As fetal anemia becomes more severe, blood viscosity drops and the heart compensates by pumping faster, which raises the peak systolic velocity above what is expected for gestational age. Comparing the observed velocity to the population median for that gestational age - expressed as a multiple of the median, or MoM - gives clinicians a reproducible, unit-independent number to track.
The formula behind the calculation
The predicted median MCA-PSV comes from the regression equation published by Mari and colleagues in the New England Journal of Medicine (2000), derived from Doppler measurements across pregnancies from roughly 18 to 40 weeks of gestation:
ln(MCA-PSV) = 2.31 + 0.04643 × GA, so predicted median PSV (cm/s) = e(2.31 + 0.04643 × GA), where GA is gestational age in completed weeks (decimals allowed).
The multiple of the median is then simply:
MoM = observed MCA-PSV ÷ predicted median MCA-PSV
This calculator performs both steps: it solves the regression for the expected median velocity at the gestational age you enter, then divides your observed reading by that expected value.
How to get an accurate reading
- Gestational age should come from confirmed dating (early first-trimester ultrasound or a certain last-menstrual-period date) - errors in dating shift the predicted median and distort the MoM.
- The Doppler angle of insonation should be as close to zero degrees as possible; measurements taken at a steep angle to the vessel systematically underestimate the true peak velocity.
- Measure at the proximal third of the MCA, just after it branches from the circle of Willis, with the fetus at rest (fetal movement and breathing can transiently raise the reading).
Putting the MoM result in context
A MoM below 1.50 is reassuring and is associated with a low probability of moderate-to-severe fetal anemia in the original validation cohort. A MoM at or above 1.50 is the threshold used clinically to prompt closer surveillance or further work-up, since it was associated with high sensitivity for detecting moderate-to-severe anemia in that study population. A single elevated reading is not a diagnosis - trends over serial scans, the clinical indication (such as red-cell alloimmunization or suspected parvovirus B19 infection), and the rest of the ultrasound findings all factor into management.
When to consult a professional
This calculator reproduces a published formula; it does not interpret your ultrasound, verify your gestational dating, or account for technical factors in how the Doppler measurement was obtained. Any MCA-PSV result - reassuring or elevated - should be reviewed with the obstetric or maternal-fetal-medicine team managing the pregnancy, since decisions about repeat testing or invasive procedures depend on the full clinical picture.