Middle Cerebral Artery Doppler Calculator

One vessel, two questions. The peak systolic velocity RISES with fetal anaemia and 1.5 multiples of the median is the published threshold; the pulsatility index FALLS with brain sparing. Getting either direction wrong gives a plausible number.

Last updated 8 Oct 2026

MCA peak systolic velocity and pulsatility index

PSV rises, PI falls

Peak systolic velocity (cm/s)Measured in the proximal third of the vessel with the angle of insonation near zero – unlike the indices this is an absolute velocity, so the angle matters – and with minimal probe pressure, which on its own lowers the pulsatility index.

End-diastolic velocity (cm/s)The velocity at the end of diastole, before the next systolic upstroke. Cerebral vasodilatation RAISES it, which LOWERS the pulsatility index – that is brain sparing. It must be above zero and below the time-averaged maximum for the indices to mean anything.

Time-averaged maximum velocity (cm/s)The mean of the traced maximum-velocity envelope over the cardiac cycle, needed only for the pulsatility index. It must lie between the end-diastolic and peak systolic velocities; outside them is a tracing error and the page returns nothing rather than a plausible index.

Median peak systolic velocity for this gestation (cm/s)From your own report or reference table, for this gestational week. The median rises steeply through pregnancy, so no single number stands in for it, and the published medians are a week-by-week table this calculator cannot interpolate. Every report prints either the expected median or the multiple of the median.

1.11multiples of the medianExample

Peak systolic velocity 50, end-diastolic 10, time-averaged maximum 25 cm/s; reference median peak systolic velocity 45 cm/s

Two indices on one vessel, moving in opposite directions

PSV MoM = measured peak systolic velocity ÷ the median for gestation — RISES with anaemia; 1.5 or more is the threshold
PI = (PSV − EDV) ÷ TAMXV — FALLS with brain sparing · RI = (PSV − EDV) ÷ PSV

the peak systolic velocity answers an anaemia question: fetal anaemia lowers blood viscosity and raises cardiac output, and the middle cerebral artery peak systolic velocity rises with it. At or above 1.5 multiples of the median, moderate to severe anaemia was detected with 100% sensitivity and a 12% false-positive rate in Mari and colleagues’ 111 at-risk fetuses
the pulsatility index answers a placental question: cerebral vasodilatation in response to reduced placental function raises the end-diastolic velocity, which LOWERS the pulsatility index. SMFM describes brain sparing as increased impedance in the umbilical arteries and decreased impedance in the middle cerebral arteries, and as increased end-diastolic flow velocity reflected by a low PI
what this page adds to the ratio page: the existing umbilical artery and cerebroplacental ratio calculator TAKES a middle cerebral pulsatility index as an input and divides it into the umbilical index. It does not compute the index from velocities and says nothing about the peak systolic velocity or about anaemia. This page does both; that page owns the ratio
why the median comes from you: the median peak systolic velocity for gestational age is a published week-by-week table and rises steeply through pregnancy. This engine cannot interpolate it and a fitted curve would misclassify at the extremes, so the figure comes from your report, which prints it. The 1.5 threshold itself is an exact published number and is implemented exactly

Worked example

Peak systolic velocity 50, end-diastolic 10, time-averaged maximum 25 cm/s; reference median peak systolic velocity 45 cm/s
PSV MoM = 50 ÷ 45 = 1.11 multiples of the median, below the 1.5 threshold
PI = (50 − 10) ÷ 25 = 40 ÷ 25 = 1.60, and RI = 40 ÷ 50 = 0.80
Raise the end-diastolic velocity to 20 cm/s, which is cerebral vasodilatation: the PI FALLS to 1.20 and the multiple of the median does not move at all. Two indices, two questions
Instead raise the peak systolic velocity to 67.5 cm/s: the multiple of the median becomes exactly 1.50, which is the value the two published readings of the threshold disagree about
Raise it to 90 cm/s and the multiple of the median is 2.00; the inverse row, 45 ÷ 90, reads 0.50 and FALLS where the headline rises
Set the end-diastolic velocity to zero and the page returns nothing: absent end-diastolic flow is reported as the finding

The indices at the default waveform, and which way each moves

The last row exists because a multiple of the median built upside down returns a number of the right magnitude that moves the wrong way. The resistance index’s bound of 0 to 1 is the quickest check on which index a report has given.
Index	Definition	Value and direction
Peak systolic velocity, multiples of the median	measured ÷ the median for gestation	1.11 — rises with anaemia
Pulsatility index	(PSV − EDV) ÷ time-averaged maximum velocity	1.60 — falls with brain sparing
Resistance index	(PSV − EDV) ÷ PSV	0.80 — falls with brain sparing, bounded 0 to 1
Systolic to diastolic ratio	PSV ÷ EDV	5.00 — falls with brain sparing
The deliberate inverse	the median ÷ the measured PSV	0.90 — falls where the headline rises

What the 1.5 multiple of the median threshold predicted

The 100% sensitivity came with a 12% false-positive rate in a population selected for alloimmunisation risk. The last row is a real disagreement about whether exactly 1.50 is over the line; this page implements the inclusive reading.
Figure	Value	Cohort
Sensitivity for moderate to severe anaemia	100%	111 fetuses at risk from red-cell alloimmunisation, against 265 normal fetuses
95% interval, non-hydropic fetuses	86 to 100%	23 non-hydropic anaemic fetuses
False-positive rate	12%	Same study
Definition of moderate anaemia	Haemoglobin below 0.65 times the median for normal fetuses	Same study
Definition of severe anaemia	Haemoglobin below 0.55 times the median	Same study
The comparator, and the two sources disagree	1.5 times the median or higher, or above 1.5 times the median	A protocol document and a secondary summary respectively

One vessel, two indices, two directions

The middle cerebral artery carries two measurements that answer different questions and move for different reasons, and keeping them apart is the job of this page. The peak systolic velocity is absolute: fetal anaemia lowers viscosity and raises cardiac output, so the velocity rises, and at or above 1.5 multiples of the median for gestational age Mari and colleagues detected moderate to severe anaemia with 100% sensitivity and a 12% false-positive rate in 111 fetuses at risk from red-cell alloimmunisation. The pulsatility index is a ratio: cerebral vasodilatation raises the end-diastolic velocity and so lowers the index. SMFM describes brain sparing as increased umbilical and decreased middle cerebral impedance, and as increased end-diastolic flow reflected by a low index.

So a rise in one is not a rise in the other. Raise the end-diastolic velocity on the default waveform from 10 to 20 cm/s and the pulsatility index falls from 1.60 to 1.20 while the multiple of the median does not move at all, because the multiple depends only on the peak systolic velocity and the reference median. The inverse row in the output is there on purpose: a multiple of the median built upside down returns a plausible number that moves the wrong way and would read a severely anaemic fetus as improving. This category has already met a published source with a ratio stated upside down, so the direction is printed next to every number.

This page does not duplicate the umbilical artery and cerebroplacental ratio page. That page takes a middle cerebral pulsatility index as an input and divides it into the umbilical index; it does not compute the index from velocities and says nothing about the peak systolic velocity or about anaemia. This one does both. What it does not do is hold a reference table: the median peak systolic velocity rises steeply with gestation, the published medians are a week-by-week table this engine cannot interpolate, and a fitted curve would misclassify at the extremes, so the median comes from your report. Two things shape the measurement itself: the peak systolic velocity is absolute, so the insonation angle matters in a way it does not for the ratios, and probe pressure on the fetal head lowers the pulsatility index on its own. The later marker in the sequence is the ductus venosus. This page computes a number, or applies published criteria, and names the body that publishes them. It renders no clinical decision: nothing here says whether this fetus is compromised, whether a tracing should be acted on, whether a scan should be repeated, whether labour should be induced or when a pregnancy should be delivered. An obstetric measurement is read alongside the whole clinical picture – history, gestational age, serial trend, the other biometry – and never instead of it. It supports a clinician’s judgement rather than replacing it. Thresholds here are the ones their named sources print. Reference standards differ between guidelines, a finding can cross a threshold purely by changing standard, and the cut-off your own unit works to takes precedence over anything on this page.

Frequently asked questions

Does the middle cerebral artery pulsatility index rise or fall in a compromised fetus?

It falls. Cerebral vasodilatation raises the end-diastolic velocity, which lowers the index: that is brain sparing. The umbilical index rises instead, which is why the cerebroplacental ratio falls on both counts. The peak systolic velocity on the same vessel rises with anaemia, which is a separate question.

How does this differ from the cerebroplacental ratio page?

That page takes a middle cerebral pulsatility index as an input and divides it into the umbilical index; it does not compute the index from velocities and does not mention the peak systolic velocity or fetal anaemia. This page computes all three indices from the velocities, and the peak systolic velocity as a multiple of the median, which is the anaemia measurement.

Is the threshold above 1.5 multiples of the median, or at or above it?

The two sources read here disagree. A radiology protocol prints it as 1.5 times the median or higher; a secondary summary of Mari’s paper states it as above 1.5 times the median. A fetus at exactly 1.50 is over the line one way and under it the other. This page implements the inclusive reading and prints the disagreement.

Why does the page ask for the median velocity?

Because a multiple of the median is meaningless without one, and the medians are a week-by-week table that rises steeply through pregnancy. This calculator cannot interpolate one and will not fit a curve to it, because a fit misclassifies at the extremes. Your report prints the median.

Does a raised peak systolic velocity mean the fetus is anaemic?

No. The 100% sensitivity came with a 12% false-positive rate in a population selected for alloimmunisation risk, so the positive predictive value is lower in a lower-risk population. It is also an absolute velocity, so the insonation angle matters and it must be taken in the proximal third of the vessel with minimal probe pressure.

Related calculators

References

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(1):9-14.
University of Washington Department of Radiology. OB Doppler Protocols, July 2025. Seattle, WA.
Society for Maternal-Fetal Medicine Publications Committee (Berkley E, Chauhan SP, Abuhamad A). Doppler assessment of the fetus with intrauterine growth restriction. Am J Obstet Gynecol. 2012;206(4):300-8.

Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/