Late Pregnancy · Fetal Anaemia
MCA-PSV — Fetal Anaemia Detection
Middle Cerebral Artery Peak Systolic Velocity detects fetal anaemia non-invasively. >1.5 MoM = moderate-severe anaemia. Used for Rh disease, parvovirus B19, TAPS twins. Intrauterine transfusion (IUT) treatment when needed.
Last reviewed 2 June 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.
What is MCA-PSV?
Middle Cerebral Artery Peak Systolic Velocity. Detects fetal anaemia: anaemic blood thinner → flows faster → higher peak velocity.
Threshold: >1.5 MoM = significant anaemia. Non-invasive alternative to historical cordocentesis.
Fetal anaemia causes
- Rh disease (maternal antibodies destroying fetal cells).
- Parvovirus B19 (9-20 weeks).
- TTTS / TAPS in monochorionic twins.
- Alpha-thalassaemia.
- Fetal-maternal haemorrhage.
- Severe abruption.
When monitored
- Maternal antibodies detected.
- Parvovirus exposure (weekly 9-20 wk).
- Twin pregnancies (TAPS).
- Fetal hydrops on ultrasound.
- Unexplained FMH.
Intrauterine transfusion (IUT)
- Ultrasound-guided needle into umbilical vein.
- Donor red cells (O-neg, CMV-neg, irradiated, fresh).
- Target Hb ~14 g/dL.
- 30-60 min procedure.
- Procedure-related fetal death ~1-3% per IUT.
- Live birth ~85-95% even in severe cases.
- UK specialist centres: King’s, UCLH, Birmingham, Newcastle.
Parvovirus B19 in pregnancy
Common cause at 9-20 weeks. Schools / nurseries outbreaks. Adult infection often mild. Fetal effects: red cell suppression → anaemia → hydrops → fetal death.
Monitoring: weekly MCA-PSV for 12 weeks post-infection. IUT if anaemia confirmed.
TAPS (Twin Anaemia Polycythaemia Sequence)
Monochorionic twins. Chronic blood imbalance without TTTS fluid changes. Detected by MCA-PSV discrepancy (one >1.5 MoM, one <1.0 MoM). Treatment: laser, selective cord coagulation, IUT for anaemic twin.
Future Rh pregnancies
- Antibodies persist + worsen.
- Titre monitoring + MCA-PSV from 18 wk (or earlier).
- IUT pathway prepared.
- cffDNA can determine baby’s Rh.
- Preconception consultation.
Different scenarios
Scenario 1: Anti-D antibodies titre rising in Rh-neg mum
Weekly MCA-PSV from 18 wk. IUT planning at specialist centre.
Scenario 2: Parvovirus exposure at 16 wk
Weekly MCA-PSV for 12 weeks. Most resolve; IUT if anaemia confirmed.
Scenario 3: MCA-PSV 1.7 MoM, hydrops developing
Cordocentesis confirmation; IUT urgent. Specialist transfer.
Scenario 4: Twin MC pregnancy, MCA-PSV 1.8 + 0.8 in twins
TAPS. Specialist intervention. Laser / cord coagulation / IUT.
Scenario 5: Post-IUT, stable, planning delivery 35 wk
Planned delivery. Steroids if needed. Neonatal team aware; postnatal transfusions may be needed.
Care guidance — MCA-PSV
- Antenatal antibody screening at booking + 28 wk.
- Anti-D prophylaxis for Rh-neg.
- cffDNA where available to determine baby’s Rh.
- Specialist fetal medicine for high-titre antibodies.
- Avoid parvovirus exposure 9-20 wk if possible.
- Monitor MCA-PSV weekly if active risk.
- IUT in specialist centres only.
- Postnatal newborn follow-up for late anaemia.
Sources
- Mari G, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. NEJM 2000.
- RCOG Green-top Guideline 65. The management of women with red cell antibodies during pregnancy.
- ISUOG Practice Guidelines. Doppler ultrasonography.
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Frequently asked questions
What is MCA-PSV?
MIDDLE CEREBRAL ARTERY PEAK SYSTOLIC VELOCITY — ultrasound Doppler measurement detecting FETAL ANAEMIA. Anaemic baby's blood is THINNER → flows FASTER through MCA → higher peak velocity. THRESHOLD: PSV >1.5 MoM (multiples of median for gestational age) = significant anaemia. EXTENSIVELY validated by Mari + colleagues. NON-INVASIVE alternative to historical CORDOCENTESIS (fetal blood sampling) for monitoring.
Why is fetal anaemia detected?
FETAL ANAEMIA causes: (1) RHESUS DISEASE (Rh-D, anti-K, others) — maternal antibodies destroy fetal red cells; (2) PARVOVIRUS B19 infection — affects fetal bone marrow; (3) TWIN-TWIN TRANSFUSION SYNDROME / TAPS in MC twins; (4) HOMOZYGOUS alpha-thalassaemia; (5) FETAL infection (other); (6) ALPHA-THALASSAEMIA major; (7) BLEEDING (fetal-maternal haemorrhage, abruption). EARLY detection prevents fetal hydrops + death.
When is MCA-PSV monitored?
(1) MATERNAL ANTIBODIES detected — Rh, anti-K, others; weekly from titre rise; (2) PARVOVIRUS B19 exposure — weekly 9-20 weeks if at risk; (3) TWIN PREGNANCIES suspected TAPS; (4) FETAL HYDROPS detected on ultrasound; (5) UNEXPLAINED feto-maternal haemorrhage. SCHEDULE: weekly when monitoring active anaemia risk; less frequent if stable.
What if MCA-PSV is high?
MCA-PSV >1.5 MoM = moderate-severe anaemia likely. CONFIRMATION often by CORDOCENTESIS (fetal blood sample) — direct measurement of haemoglobin. TREATMENT: INTRAUTERINE BLOOD TRANSFUSION via umbilical vein (IUT) — donor red cells given to fetus. CAN BE LIFESAVING. PROCEDURE: ultrasound-guided needle through mum's abdomen, into umbilical vein at placenta insertion site; baby paralysed briefly with neuromuscular blocker; blood transfused. REPEATED at intervals (every 1-4 weeks) until baby can be delivered safely. SPECIALIST fetal medicine centres only.
What about intrauterine transfusion (IUT)?
LIFE-SAVING procedure for severe fetal anaemia. PROCEDURE: ULTRASOUND-GUIDED needle into umbilical vein at placental insertion. DONOR RED CELLS (O-negative, CMV-negative, irradiated, leukoreduced, fresh) infused. VOLUME calculated to bring fetal Hb to ~14 g/dL. DURATION 30-60 min. RISKS: ~1-3% procedure-related fetal death per IUT; bleeding, infection, premature labour. SUCCESS RATES: ~85-95% live birth even in severe cases. SPECIALIST CENTRES UK: e.g. King's, UCLH, Birmingham, Newcastle.
Could parvovirus B19 cause this?
YES — common cause in pregnancy at 9-20 WEEKS. PARVOVIRUS B19 ('slapped cheek' / fifth disease) — common in children; school outbreaks. ADULT INFECTION often mild. FETAL EFFECTS: red blood cell suppression → severe anaemia → hydrops fetalis → fetal death. SCREENING: mothers with confirmed exposure / infection — weekly MCA-PSV scan for 12 weeks post-infection. ELEVATED PSV: IUT considered. RESOLUTION usually with treatment. AFTER 20 WEEKS: risk much lower (mature fetal bone marrow).
Does this affect this pregnancy long-term?
VARIES. EARLY HYDROPS / SEVERE anaemia treated successfully: most children develop normally. RECURRENT IUT pregnancies: increased risk of preterm birth, growth restriction. NEUROLOGICAL outcomes generally good with appropriate treatment. POSTNATAL: follow-up monitoring for late anaemia, jaundice management, sometimes ongoing transfusions or stem cell support (severe Rh disease).
Will my future pregnancies need monitoring?
RH DISEASE: yes — antibodies persist; severity often worsens with each pregnancy. NEXT pregnancy: titre monitoring; MCA-PSV from 18 weeks (or earlier if titres high); IUT pathway prepared. PRECONCEPTION CONSULTATION valuable. PARTNER blood type discussion. ALTERNATIVE: IVF + PGT-M with sperm donor (different blood group) — extreme cases. RHIG (anti-D Ig) prevention in future Rh-negative non-sensitised mothers.
What if I'm carrying twins with anaemia concern?
MONOCHORIONIC twins — TWIN ANAEMIA POLYCYTHAEMIA SEQUENCE (TAPS): chronic blood imbalance without classic TTTS fluid changes. ONE TWIN anaemic, OTHER polycythaemic. DETECTED by MCA-PSV discrepancy between twins (one >1.5 MoM, one <1.0 MoM). TREATMENT: laser ablation; selective cord coagulation; IUT for anaemic twin. SPECIALIST fetal medicine care.
How accurate is MCA-PSV?
GOOD — >90% sensitivity for moderate-severe anaemia, ~85% specificity. CORRELATES well with actual haemoglobin. INTERVAL between measurements important (sometimes anaemia develops between scans). COMBINED with clinical context, biophysical profile, growth measurements informs decisions. NOT perfect but vastly better than older approaches (cordocentesis at lower threshold = more procedures + risks).
Will I have a normal pregnancy after IUT?
MONITORED CLOSELY. ROUTINE: weekly MCA-PSV; monthly growth + Doppler; CTG. DELIVERY: usually planned 34-37 weeks (depends on stability). VAGINAL BIRTH possible if otherwise low-risk. NEONATAL TEAM aware; possible postnatal transfusion/exchange transfusion needed; phototherapy for jaundice common. RECOVERY: variable; most babies do well long-term.
What does my partner need to know about Rh disease?
(1) PARTNER'S Rh-D status if mum Rh-neg — informs likelihood baby Rh-positive; (2) HETEROZYGOUS vs homozygous Rh-pos father — affects baby's Rh probability (cffDNA test now resolves directly); (3) FUTURE PREGNANCIES — anti-D antibodies persist + worsen; partner blood type relevant. SUPPORT for partner — IUT pregnancies stressful. KNOWING involved means better preparation.
How does this relate to other calculators on BumpBites?
Companion: /calculators/anti-d-dosing for Rh prevention; /calculators/blood-type; /calculators/kleihauer-betke; /calculators/cpr-doppler; /calculators/ua-dv-doppler; /calculators/fgr-doppler-composite; /calculators/biophysical-profile; /calculators/ttts-quintero.