Late Pregnancy · Fetal Doppler
CPR — Cerebroplacental Ratio Doppler
Ratio of MCA-PI to UA-PI on fetal Doppler. Low CPR = brain-sparing pattern = placental insufficiency. Sensitive marker for late-onset growth restriction. Used in IUGR, post-dates, twins, PE, diabetes. NICE NG137 / ISUOG.
Last reviewed 2 June 2026
Cerebroplacental ratio (CPR)
MCA PI / UA PI — brain-sparing index
MCA-PI
UA-PI
Enter MCA pulsatility index and UA pulsatility index.
Educational tool only — not medical advice. CPR < 1.0 indicates brain-sparing (MCA PI ≤ UA PI) — predictor of adverse outcome even in apparently adequately-grown fetuses at term. ISUOG / SMFM increasingly include CPR in FGR surveillance.
What does this mean?
CPR — the ratio of middle cerebral artery PI to umbilical artery PI — captures the fetal “brain-sparing” response to placental insufficiency. When the placenta fails, the fetus preferentially shunts blood to the brain, dropping MCA PI (cerebral vasodilation) while UA PI rises (peripheral vasoconstriction). When the ratio falls below 1.0 (some references use the 5th percentile for GA), the brain-sparing reflex is active — an adverse-outcome predictor even in apparently normally-grown (AGA) fetuses at term. CPR is increasingly used alongside UA Doppler and EFW trajectory in late-onset FGR surveillance(after 32–34 wk), where the classic AEDF/REDF progression is uncommon. Used in ISUOG, SMFM, RCOG FGR pathways. Repeat weekly, sooner if any concerning change.
What is CPR?
Cerebroplacental Ratio = MCA-PI ÷ UA-PI. Lower CPR = placental insufficiency. Sign of fetal redistribution (brain-sparing) when placenta failing.
When is Doppler done?
- Growth restriction (IUGR/SGA).
- Pre-eclampsia.
- Twin pregnancy surveillance.
- Post-dates.
- Reduced fetal movements.
- Previous stillbirth.
- Autoimmune (lupus, APS).
- Diabetes.
- Maternal renal disease.
What CPR predicts
Low CPR associated with: adverse neonatal outcomes (NICU, low Apgar, acidosis), stillbirth risk (especially late pregnancy), confirmed growth restriction. Even in normal-sized babies can signal compromise.
Brain-sparing
Adaptive fetal response. Baby redistributes blood preferentially to brain when placenta insufficient. MCA Doppler reduces; precedes other decompensation signs — warning to plan delivery.
Delivery considerations
- Low CPR at term (>37 wk): delivery often recommended.
- Low CPR preterm: balance risks; antenatal steroids; magnesium if <32 wk.
- Multiple Doppler abnormalities (UA AEDF/REDF, DV a-wave reversal): more urgent.
Other Dopplers
- UA (umbilical artery): placental resistance; AEDF / REDF concerning.
- MCA (middle cerebral): brain perfusion; low PI = brain-sparing; high PSV = anaemia.
- Uterine artery: maternal side; high resistance = PE / IUGR risk.
- Ductus venosus (DV): cardiac; a-wave reversal = late-stage.
Is Doppler safe?
Yes — standard pregnancy ultrasound. No radiation. Slightly higher energy than B-mode; ALARA principle limits exposure. Multiple scans fine clinically.
Frequency in high-risk pregnancies
- Stable: weekly.
- Declining: 2-3x weekly.
- Severe: daily + possible admission.
Different scenarios
Scenario 1: 36 wk IUGR, EFW <10th centile, CPR low
Delivery often recommended. Antenatal steroids if needed. Plan within 24-48h.
Scenario 2: 30 wk severe IUGR, AEDF + low CPR
Admission. Steroids. Magnesium. Daily Dopplers + BPP. Deliver as soon as steroids effective.
Scenario 3: Twin pregnancy, CPR borderline for one twin
Specialist fetal medicine. Closer monitoring. Plan delivery 34-37 wk based on trajectory.
Scenario 4: Post-dates 41+5, CPR low, otherwise reassuring
Induction within 24-48h. Continuous CTG in labour.
Scenario 5: Reduced movements + normal CPR + normal CTG
Reassuring. Continue movement awareness. Routine follow-up.
Care guidance — CPR Doppler
- For high-risk pregnancies, not routine.
- Combine with EFW, BPP, CTG.
- Serial tracking matters.
- Low CPR informs delivery timing.
- Watch fetal movements daily.
- Specialist fetal medicine input.
- Mental health support — anxiety common in monitored pregnancies.
Sources
- NICE NG137. Twin and triplet pregnancy.
- ISUOG Practice Guidelines. Use of Doppler ultrasonography in obstetrics.
- RCOG Green-top 31. Investigation and management of the small-for-gestational-age fetus.
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Frequently asked questions
What is the cerebroplacental ratio (CPR)?
RATIO of two Doppler measurements: MIDDLE CEREBRAL ARTERY PULSATILITY INDEX (MCA-PI) ÷ UMBILICAL ARTERY PULSATILITY INDEX (UA-PI). HIGHER CPR = better placental function. LOW CPR (<5th centile or <1.0 absolute) = sign of FETAL DISTRESS / placental insufficiency. CONCEPT: baby in distress redirects blood to brain ('brain sparing') — drops MCA resistance + may raise UA resistance. PROVIDES SENSITIVE marker of compromise BEFORE other signs. USED IN: growth restriction, post-dates, twin pregnancies.
Why is my baby having Doppler scans?
INDICATIONS: (1) GROWTH RESTRICTION (IUGR / SGA — small for gestational age); (2) PRE-ECLAMPSIA / hypertensive disorders; (3) TWIN pregnancy surveillance; (4) POST-DATES; (5) REDUCED FETAL MOVEMENTS; (6) PREVIOUS UNEXPLAINED STILLBIRTH; (7) AUTOIMMUNE DISEASE (lupus, APS); (8) DIABETES; (9) MATERNAL renal disease. ROUTINE LOW-RISK pregnancies don't have Doppler. PREGNANCY MONITORING tool — guides timing of delivery.
Does CPR predict outcomes?
YES — moderate. LOW CPR associated with: (1) ADVERSE neonatal outcomes (NICU admission, low Apgar, acidosis); (2) STILLBIRTH risk (especially late pregnancy); (3) GROWTH RESTRICTION confirmation. EVEN IN APPARENTLY NORMAL-sized babies, low CPR signals compromise. INFORMS delivery timing decisions — sometimes triggers earlier delivery to prevent stillbirth. INTERPRETED with other Dopplers + biophysical profile + clinical context.
How is CPR measured?
TRANSABDOMINAL ULTRASOUND. (1) MCA: probe targets middle cerebral artery at brain skull base; Doppler measurement of pulsatility index. (2) UA: at free loop of umbilical cord; same measurement. (3) CALCULATED: MCA-PI ÷ UA-PI. (4) PLOTTED on gestational age centiles. NO PREP needed; takes ~15-30 minutes for full Doppler study. NON-INVASIVE; safe; no harm to baby.
What's brain-sparing?
ADAPTIVE RESPONSE in fetus when placenta isn't delivering enough oxygen. BABY REDISTRIBUTES blood flow PREFERENTIALLY TO BRAIN (vital organ). MCA Doppler shows REDUCED resistance (more blood flow to brain). CONCERNING because: indicates compromise; can't be maintained indefinitely; PRECEDES other decompensation signs (heart, body); WARNING SIGN to plan delivery. BRAIN-SPARING in CPR detection: MCA-PI low + UA-PI normal/high = CPR ratio falls.
When is delivery considered?
(1) LOW CPR at TERM (>37 wk): delivery often recommended; outcomes better than continued monitoring. (2) LOW CPR PRETERM: depends on gestation + other findings; balance risk of preterm delivery vs continued in-utero stress. (3) ADDITIONAL DOPPLER abnormalities (umbilical artery absent end-diastolic flow, reverse flow, ductus venosus a-wave reversal): more urgent. (4) BIOPHYSICAL PROFILE + CTG abnormalities. RCOG / NICE NG137 guide stratified approach.
What if CPR keeps falling?
PROGRESSION concerning. SERIAL scanning shows trajectory. STEADY decline: discussion about delivery timing. SHARP DROP: usually triggers delivery (especially term / late preterm). ANTENATAL STEROIDS if preterm. MAGNESIUM SULPHATE if <32 wk. NEONATAL TEAM prepared. EACH CASE individualised. SHARED DECISION with parents.
What are other Dopplers measured?
(1) UMBILICAL ARTERY (UA) — placental resistance; abnormal: high resistance, absent end-diastolic flow (AEDF), reversed end-diastolic flow (REDF — pre-terminal); (2) MIDDLE CEREBRAL ARTERY (MCA) — brain perfusion; low PI = brain-sparing; high PSV = fetal anaemia; (3) UTERINE ARTERY — maternal side placenta; high resistance = PE / IUGR risk; (4) DUCTUS VENOSUS (DV) — cardiac compromise marker; abnormal A-wave = late-stage decompensation. ALL COMBINED gives picture.
Is the test safe?
YES — standard pregnancy ultrasound. No radiation; no biological harm at diagnostic intensities. DOPPLER ULTRASOUND uses slightly higher energy than B-mode (image); precautions limit exposure (ALARA principle — As Low As Reasonably Achievable). RESPONSIBLE use during established pregnancy; especially second + third trimester. SAFE for baby + mother. MULTIPLE SCANS through pregnancy fine in clinical context.
How accurate is CPR?
MODERATE — informs decisions but not absolute. SENSITIVITY ~30-50% for adverse outcomes; SPECIFICITY ~70-80%. COMBINED with other markers (BPP, EFW, UA, MCA, DV, CTG) more accurate than alone. CONTEXT matters: gestational age, growth trajectory, maternal condition. EARLY-onset growth restriction (<32 wk) — UA Doppler more important; LATE-onset growth restriction — CPR more useful.
Why does the team scan so often?
HIGH-RISK pregnancies: weekly or twice-weekly Doppler + BPP + CTG. RAPID changes possible in placental insufficiency. SERIAL TRACKING > single snapshot. Allows EARLY DETECTION of deterioration → timely delivery → better outcomes. INTENSITY of monitoring varies — most stable: weekly; declining: 2-3x weekly; severe: daily + admission for monitoring.
What if I want fewer scans?
DISCUSS with consultant. SCANS for benefit not entertainment — purpose is detecting compromise to inform delivery. DECLINING scans: increased uncertainty about timing. INFORMED CHOICE — risks explained. SOME women find frequent scans stressful; team can support emotionally. OPTIONS: longer intervals; combine with movement awareness + CTG. SHARED decision.
What can I do at home?
MOST IMPORTANT: FETAL MOVEMENT awareness. CALL maternity immediately if reduced / absent movements. KEEP scan appointments. EAT well; gentle activity; sleep. MONITOR for symptoms (pre-eclampsia signs if applicable: BP, urine). FOLLOW any medication / aspirin plan. PARTNER support important. MENTAL HEALTH — anxiety high in monitored pregnancies; perinatal mental health support available.
How does this relate to other calculators on BumpBites?
Companion: /calculators/mca-psv; /calculators/ua-dv-doppler; /calculators/fgr-doppler-composite; /calculators/biophysical-profile; /calculators/kick-counter; /calculators/fetal-weight; /calculators/cervical-length; /calculators/antenatal-steroids; /calculators/magnesium-sulphate.