Pregnancy · Fetal surveillance
MCA-PSV for Fetal Anaemia
Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV) MoM calculator (Mari NEJM 2000) — non-invasive Doppler screen for moderate-to-severe fetal anaemia. Used in Rh alloimmunisation, parvovirus B19 fetal infection, monochorionic twin TAPS.
Last reviewed 25 May 2026
MCA peak systolic velocity — fetal anaemia screen
MCA-PSV (Mari 2000)
wk
cm/s
Enter gestational age and measured MCA peak systolic velocity to calculate MoM.
Educational tool only — not medical advice. MCA-PSV measurement requires an optimal angle of insonation (close to 0°), proximal MCA, and trained sonographer. Mari’s 1.5 MoM threshold has 88 % sensitivity / 82 % specificity for moderate-to-severe fetal anaemia. Used in Rh alloimmunisation, ABO incompatibility with high titres, parvovirus B19 fetal infection, and monochorionic twin TAPS.
What does this mean?
MCA-PSV transformed fetal anaemia management. Before Mari’s 2000 NEJM paper, suspected fetal anaemia (Rh alloimmunisation, parvovirus B19, monochorionic twin TAPS) needed serial amniocenteses with ΔOD450 spectrophotometry — invasive and risky. Mari showed a non-invasive Doppler measurement could detect moderate–severe anaemia with 88 % sensitivity / 82 % specificity at ≥ 1.5 multiples of the median (MoM) for gestational age. Mechanism: as fetal haematocrit falls, viscosity drops and peak systolic velocity rises. Today MCA-PSV is the first-line test, followed by intrauterine transfusion (IUT) if severe. Note: above ~35 weeks specificity falls (false positives more common); the threshold is for the mid-trimester surveillance window. Best measurements: 0° angle of insonation, proximal MCA close to its origin, trained sonographer.
Introduction
The Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV) is a non-invasive Doppler ultrasound test for fetal anaemia. Mari 2000 (NEJM) demonstrated that an MCA-PSV value at or above 1.5 multiples of the median (MoM) for gestational age predicts moderate-to-severe fetal anaemia with 88 % sensitivity and 82 % specificity — replacing the older invasive amniocentesis with ΔOD450 spectrophotometry as the first-line investigation.
Why MCA-PSV works
In fetal anaemia, the blood is less viscous (fewer red cells), so it flows faster. The fetal heart also outputs more to compensate. The middle cerebral artery — perfusing the brain at high priority — shows the clearest velocity response. The peak systolic velocity rises with the severity of anaemia.
When MCA-PSV is indicated
- Rh alloimmunisation with critical anti-D, anti-c, or anti-Kell titres.
- ABO incompatibility with high titres.
- Parvovirus B19 fetal infection (peak risk 4-6 weeks post maternal infection).
- Monochorionic twin pregnancy — Twin Anaemia-Polycythaemia Sequence (TAPS), donor twin.
- Fetal hydrops of unknown cause.
- Fetomaternal haemorrhage suspected.
Measurement technique
- Identify the proximal MCA near its origin from the internal carotid (Circle of Willis).
- Minimise angle of insonation — target 0°, ≤ 15° acceptable.
- Avoid maternal head movement and fetal breathing artefacts.
- Measure the highest velocity peak in 3 cardiac cycles.
- Record the highest of 3 measurements.
- Calculate MoM against the gestational-age-specific median (Mari).
Interpretation
- < 1.3 MoM — within normal range. Continue surveillance per indication.
- 1.3-1.5 MoM — borderline. Repeat in 1 week. Watch trajectory and other anaemia signs (hydrops, cardiomegaly, ascites).
- ≥ 1.5 MoM — moderate-to-severe fetal anaemia probable. Refer to fetal medicine for cordocentesis and intrauterine transfusion decision.
Management at ≥ 1.5 MoM
Cordocentesis (percutaneous umbilical blood sampling) under ultrasound guidance confirms the diagnosis (haemoglobin / haematocrit measurement) and allows intrauterine transfusion in the same procedure if needed (haematocrit < 30 %). Donor red cells (group O, Rh-negative, CMV-negative, irradiated) are transfused via the umbilical vein. Subsequent transfusions are often needed at 2-3 week intervals depending on the underlying cause.
Repeat schedule by indication
- Rh alloimmunisation — every 1-2 weeks from 18-20 weeks; more frequent as 1.5 MoM threshold is approached.
- Parvovirus B19 — weekly for 6-8 weeks after maternal infection.
- Monochorionic TAPS — every 2 weeks from 16 weeks.
- Post-intrauterine transfusion — every 7-10 days.
Limitations
- Operator-dependent — trained sonographers and standardised technique are essential.
- False-negative rate 12 %; false-positive rate 18 %.
- Less reliable after intrauterine transfusion (donor blood has different rheology than fetal blood).
- Less reliable at > 35 weeks gestation — alternative measures (delivery + neonatal exchange) often used.
- This educational tool implements Mari 2000 medians; some centres use slightly different population-specific medians.
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.
- ACOG. Practice Bulletin 192: Management of Alloimmunization During Pregnancy.
- ISUOG Practice Guidelines: Use of Doppler ultrasonography in obstetrics.
- RCOG Green-top Guideline 65: Management of Women with Red Cell Antibodies during Pregnancy.
- Moise KJ. Management of rhesus alloimmunization in pregnancy. Obstet Gynecol 2008;112:164-76.
Frequently asked questions
What is MCA-PSV?
Middle Cerebral Artery Peak Systolic Velocity — the peak velocity of blood flow measured in the fetal middle cerebral artery using Doppler ultrasound. In fetal anaemia, the blood is less viscous (fewer red cells), so it flows faster — measurably faster in the MCA. Mari 2000 (NEJM) showed that an MCA-PSV ≥ 1.5 multiples of the median (MoM) for gestational age predicts moderate-to-severe fetal anaemia with 88 % sensitivity and 82 % specificity. It replaced amniocentesis with ΔOD450 spectrophotometry as the first-line investigation.
When is MCA-PSV measured?
Indications: Rh alloimmunisation with anti-D, anti-c, or anti-Kell antibodies (titre ≥ 1:16 anti-D, or any anti-Kell); ABO incompatibility with high titres; parvovirus B19 fetal infection (peaks 4-6 weeks post-maternal infection); monochorionic twin pregnancy with twin anaemia-polycythemia sequence (TAPS) — donor twin; fetal hydrops of unknown cause; fetomaternal haemorrhage suspected. Typically performed weekly to fortnightly from 18-20 weeks until risk resolves.
How is MCA-PSV measured correctly?
Strict technique: identify the proximal MCA close to its origin from the internal carotid (Circle of Willis); minimise angle of insonation (target 0°; ≤ 15° acceptable); avoid maternal head movement and fetal breathing artefact; measure the highest velocity peak in 3 cardiac cycles; record the highest of 3 measurements. Sonographer training matters — inter-observer variability is 5-15 %.
Why MCA and not other vessels?
The fetal MCA is large enough to insonate easily, has a relatively constant angle of measurement (head is engaged), and shows the clearest velocity response to fetal anaemia because it perfuses the brain at high priority. Other vessels (umbilical, ductus venosus, descending aorta) have been studied; MCA-PSV outperformed them all in Mari's foundational study.
What's the management threshold?
MCA-PSV ≥ 1.5 MoM → refer to fetal medicine for cordocentesis (fetal blood sampling). Cordocentesis confirms the diagnosis (haemoglobin / haematocrit measurement) and allows intrauterine transfusion in the same procedure if anaemia is moderate-to-severe (haematocrit < 30 %). Transfusion is via the umbilical vein under ultrasound guidance.
How often is screening repeated?
Depends on the underlying indication. Rh-alloimmunisation: typically every 1-2 weeks from 18-20 weeks; more often once MCA-PSV approaches 1.5 MoM. Parvovirus: weekly for 6-8 weeks after maternal infection. TAPS in monochorionic twins: every 2 weeks from 16 weeks. After intrauterine transfusion: every 7-10 days because subsequent transfusions are often needed at 2-3 week intervals.
Is the test perfect?
No — 12 % false-negative rate (severe anaemia missed) and 18 % false-positive rate (treatment not actually needed). For this reason MCA-PSV is interpreted in clinical context — antibody titres, prior pregnancy history, fetal hydrops signs, growth, and movement all factor in. Borderline values (1.3-1.5 MoM) usually mean repeat in 1 week rather than immediate intervention.