MCA-PSV Calculator - Fetal Anemia Risk
Enter the gestational age in weeks and the Doppler-measured middle cerebral artery peak systolic velocity (MCA-PSV) to calculate the multiple of the median (MoM). A value at or above 1.5 MoM is the widely accepted threshold for investigating moderate-to-severe fetal anemia. The reference formula is from Mari et al. (New England Journal of Medicine, 2000), the landmark study that validated non-invasive MCA Doppler screening.
What is MCA-PSV and why does it matter in pregnancy?
The middle cerebral artery (MCA) is one of the largest arteries supplying blood to the fetal brain. When a fetus becomes anemic, its blood becomes less viscous and the heart pumps faster to maintain oxygen delivery. These changes cause blood to move more rapidly through the cerebral vessels, raising the peak systolic velocity measured by Doppler ultrasound. Monitoring MCA-PSV gives obstetricians a non-invasive window into fetal blood status, replacing the older practice of routine invasive cordocentesis for all at-risk pregnancies. The technique was validated in a landmark 2000 study by Mari and colleagues published in the New England Journal of Medicine, which showed that a threshold of 1.5 multiples of the median (MoM) detected moderate-to-severe anemia with approximately 100% sensitivity and a false-positive rate of about 12%.
How to measure MCA-PSV correctly
Accurate measurement requires a standardized Doppler technique. The sample gate should be placed within 2 mm of the MCA origin from the internal carotid artery, visualized in an axial plane of the fetal head. The ultrasound beam angle must be as close to 0 degrees as possible relative to the direction of blood flow: even a 20-degree angle can underestimate velocity by about 6%. The MCA should occupy more than 50% of the magnified image. Minimum pressure should be applied to avoid fetal bradycardia. The maximum velocity over at least three waveform cycles is recorded. Proper technique and angle correction are critical, and measurements taken at non-zero angles should be treated with caution.
Interpreting MoM results and clinical thresholds
MoM expresses the observed PSV as a ratio of the expected median for that gestational age, making values comparable across weeks. A MoM below 1.0 is reassuring. Values between 1.0 and 1.29 are mildly elevated and typically require a repeat scan in one to two weeks. Values between 1.29 and 1.5 are moderately elevated and warrant closer surveillance, often within one week. A MoM at or above 1.5 - the standard action threshold - is strongly associated with moderate-to-severe fetal anemia and typically prompts referral for cordocentesis and possible intrauterine transfusion. Clinical context is essential: the cause of anemia risk (red cell alloimmunization, parvovirus B19, fetomaternal hemorrhage, twin anemia-polycythemia sequence) influences how aggressively borderline values are managed.
Conditions that elevate MCA-PSV and limitations
MCA-PSV rises in any condition that reduces fetal blood viscosity or increases cardiac output, not only classic Rh alloimmunization. Parvovirus B19 infection, fetomaternal hemorrhage, fetal hydrops from non-immune causes, placental chorioangioma, and twin-to-twin transfusion syndrome can all increase MCA-PSV. After intrauterine transfusion, fetal red cells are adult-type and the relationship between PSV and hemoglobin shifts; MCA-PSV is less reliable after a first transfusion and many centres repeat cordocentesis at fixed intervals rather than relying solely on Doppler. Importantly, false-positive rates rise after 35 weeks, so the threshold of 1.5 MoM requires more cautious interpretation in late pregnancy and should always be combined with clinical findings.
MCA-PSV reference values by gestational age (Mari et al. 2000)
| Gestational age (weeks) | Median MCA-PSV (cm/s) | 1.5 MoM threshold (cm/s) | Clinical note |
|---|---|---|---|
| 16 | 23.2 | 34.9 | Early second trimester |
| 18 | 25.5 | 38.3 | |
| 20 | 28.1 | 42.2 | |
| 22 | 31.0 | 46.5 | |
| 24 | 34.1 | 51.2 | Optimal range for monitoring |
| 26 | 37.5 | 56.3 | |
| 28 | 41.3 | 62.0 | |
| 30 | 45.5 | 68.3 | |
| 32 | 50.1 | 75.2 | |
| 34 | 55.2 | 82.8 | |
| 35 | 57.9 | 86.8 | False positives increase after 35 wks |
| 36 | 60.7 | 91.1 | |
| 38 | 66.9 | 100.3 | |
| 40 | 73.6 | 110.5 | Late-term; caution with threshold interpretation |
Expected median MCA-PSV and the 1.5 MoM action threshold at selected gestational ages. Values calculated from the formula: Median = exp(2.31 + 0.04643 x GA weeks).
Frequently asked questions
What does MoM mean in an MCA scan?
MoM stands for "multiple of the median." It is the ratio of your baby's measured MCA-PSV to the expected median value for the same gestational age. A MoM of 1.0 means exactly average; 1.5 means 50% above the median, which is the standard threshold used to flag possible moderate-to-severe fetal anemia.
Why is 1.5 MoM the action threshold?
The 1.5 MoM cut-off was established by Mari et al. (NEJM 2000) in a prospective study of 111 fetuses at risk of anemia from red cell alloimmunization. At that threshold, sensitivity for detecting a hemoglobin deficit greater than 5 g/dL was approximately 100%, with a false-positive rate of about 12%. Thresholds above 1.5 increase specificity but miss some affected fetuses, while lower thresholds greatly increase unnecessary invasive procedures.
Can MCA-PSV be used after an intrauterine transfusion?
MCA-PSV is less reliable after the first intrauterine transfusion. The transfused adult red cells behave differently from fetal cells, and the normal relationship between PSV and hemoglobin no longer holds well. Most centres switch to fixed-interval cordocentesis (typically every 2-4 weeks) to monitor fetal hemoglobin after transfusion rather than relying on MCA Doppler alone.
Does beam angle affect the measurement?
Yes, beam angle is critical. Doppler correctly estimates velocity only when the angle between the beam and blood flow is 0 degrees. A 20-degree angle introduces approximately a 6% underestimate, and larger angles cause greater errors. Measurements should always be taken as close to 0 degrees as possible without angle correction, because angle correction introduces its own imprecision at the very small angles used in MCA interrogation.
Is MCA-PSV accurate in twin pregnancies?
MCA-PSV is routinely used in twin pregnancies, particularly to screen for twin anemia-polycythemia sequence (TAPS), where the donor twin has a low MCA-PSV and the recipient twin has a high MCA-PSV. The same 1.5 MoM action threshold is generally applied, but some guidelines use more conservative thresholds (such as 1.29 MoM for the donor) when TAPS is suspected. Expert review is especially important in the twin setting.
Why are false positives more common after 35 weeks?
The underlying biology is not fully understood, but near-term and post-term fetuses show greater variability in MCA-PSV relative to the reference curve, possibly reflecting changes in fetal cardiovascular physiology and brain development. This means a value at or near 1.5 MoM in a 36- to 40-week pregnancy is more likely to be a false alarm than the same value at 24-30 weeks. Clinical context, including the indication for monitoring and fetal wellbeing assessment, becomes especially important late in pregnancy.
What causes fetal anemia that MCA-PSV is used to detect?
The most common cause is red cell alloimmunization (Rh disease), where maternal antibodies cross the placenta and destroy fetal red cells. Other causes include parvovirus B19 infection (which suppresses fetal red cell production), fetomaternal hemorrhage, fetal hydrops from various causes, twin-to-twin transfusion syndrome, alpha-thalassemia major, and placental chorioangioma. MCA-PSV monitoring is used in all of these settings as a non-invasive screen before deciding whether invasive testing is needed.