MCA Calculator

How to use this MCA-PSV calculator

What this calculator does

This calculator standardizes a fetal middle cerebral artery peak systolic velocity (MCA-PSV) by gestational age. It estimates the expected median MCA-PSV for the selected week, divides the measured Doppler velocity by that median, and reports the result as a Multiple of the Median (MoM). MoM makes a velocity easier to compare with gestational-age-specific reference values. The model is intended to support discussion of fetal-anemia screening; it does not establish a diagnosis, determine a treatment plan, replace a complete ultrasound examination, or account for every clinical circumstance.

When to use it

Typical uses include checking a reported MCA Doppler value from a pregnancy already being evaluated for fetal anemia risk, reproducing a MoM calculation during clinical education, comparing serial measurements on a consistent scale, or reviewing how a measured velocity relates to the commonly cited 1.5 MoM screening threshold. The Society for Maternal-Fetal Medicine guideline on the fetus at risk for anemia describes MCA-PSV as the primary Doppler technique for detecting fetal anemia in appropriately selected at-risk pregnancies and discusses the 1.5 MoM threshold in clinical management.

How to calculate

The calculator opens with a ready-to-use demonstration: 28 weeks and an MCA peak systolic velocity of 50.00 cm/s. Its results and Excel workbook are available immediately. To use your own values:

1. Select the completed Gestational age week. This implementation uses the supported 18-40 week range.
2. Replace MCA peak systolic velocity with the Doppler measurement in cm/s. Use a period for decimals; for example, enter 45.7, not 45,7.
3. Read the Multiple of the Median (MoM), the calculated Median peak systolic velocity, the 1.5 MoM reference threshold, and the screening-context interpretation. Results update as soon as the input changes.
4. Choose Download Excel to export the current validated inputs and calculated values as an XLSX workbook. Choose Reset to clear the demonstration and entered data. After Reset, Excel export stays disabled until both required inputs form a complete valid state again.

Input guide

Gestational age is required and is entered as a completed week from 18 through 40. A realistic example is 28 weeks. Increasing gestational age increases the model's expected median MCA velocity, so the same measured PSV produces a lower MoM at a later week. A common interpretation mistake is to compare a measured velocity with a single fixed cutoff instead of a gestational-age-adjusted reference.

MCA peak systolic velocity is required, measured in centimeters per second, and must be a finite positive decimal. A realistic example is 50.00 cm/s. At a fixed gestational age, increasing the measured PSV increases MoM proportionally. Enter the actual peak systolic velocity reported from the Doppler examination rather than a resistance index, pulsatility index, mean velocity, or a value in a different unit. Measurement technique matters: the SMFM guideline recommends sampling the MCA near its origin and ideally using a zero-degree insonation angle without angle correction.

Output guide

Multiple of the Median (MoM) is the measured PSV divided by the expected median at the selected week. It is dimensionless and displayed to two decimals. A value of 1.00 means the measured velocity equals the model median; values above 1.00 are proportionally higher than the median. The Median peak systolic velocity is the reference-model estimate in cm/s. The 1.5 MoM reference threshold converts 1.5 times that median back into cm/s so the threshold can be viewed in the same unit as the Doppler measurement. The screening interpretation states only whether the calculated MoM is below or at/above 1.5; it is not a diagnosis. The original Mari et al. multicenter study of MCA Doppler for fetal anemia established the clinical evidence base for using increased MCA-PSV in at-risk fetuses.

Worked example

With the startup example of 28 weeks and a measured MCA-PSV of 50.00 cm/s, the model calculates the median as e^(2.31 + 0.046 × 28) = 36.53 cm/s after display rounding. MoM is then 50.00 ÷ 36.53 = 1.37. The corresponding 1.5 MoM velocity is 36.53 × 1.5 = 54.79 cm/s. Therefore, the example measurement is below the 1.5 MoM screening threshold. These exact startup values are also the initial workbook checkpoints.

Learn more

For technique and measurement variability, review the PubMed record for Middle cerebral artery peak systolic velocity: technique and variability. Accurate Doppler acquisition is essential because angle, sampling location, fetal activity, and technical factors can affect velocity. If an MCA-PSV result is being used in an actual pregnancy, interpret it with the clinician responsible for the ultrasound and the indication for surveillance rather than relying on a calculator result in isolation.