UCA Doppler study
Cerebroplacental Ratio (CPR) & normal value
Normal Value
CPR (Cerebroplacental Ratio) →
Cerebroplacental Ratio (CPR) is the ratio of the pulsatility index (PI) of the middle cerebral artery (MCA) to the pulsatility index (PI) of the umbilical artery (UA). It is a composite Doppler index that compares cerebral vascular resistance with placental vascular resistance, providing a single number that reflects the balance of blood flow distribution between the fetal brain and the placenta.
Formula:
CPR = MCA PI ÷ UA PI
What it measures:
CPR assesses whether the fetus is preferentially shunting blood toward the brain at the expense of the placenta — a phenomenon known as "brain-sparing".
In a healthy fetus with a well-functioning placenta:
• The UA has low resistance (low UA PI) → blood flows easily to the placenta.
• The MCA has moderate-to-high resistance (higher MCA PI) → the brain does not need to dilate its vessels because oxygenation is adequate.
• Therefore CPR is > 1.0 (MCA PI > UA PI) — this is normal.
In a fetus with placental insufficiency and hypoxia:
• The UA has high resistance (high UA PI) → reduced placental flow.
• The MCA dilates to increase cerebral blood flow (low MCA PI) → the brain attempts to protect itself from hypoxia.
• Therefore CPR falls below 1.0 (MCA PI < UA PI) — this is brain-sparing and indicates fetal compromise.
Why CPR is important:
• CPR can become abnormal before either the UA PI or MCA PI alone crosses their respective 95th or 5th centile individually.
• It detects the redistribution phase of fetal adaptation to hypoxia — the brain dilates before the UA resistance becomes critically high.
• A low CPR is associated with adverse perinatal outcomes including IUGR, neonatal intensive care unit (NICU) admission, low Apgar scores, and long-term neurodevelopmental impairment.
• CPR is more sensitive than UA Doppler alone for detecting early fetal compromise, especially in late-onset FGR (fetal growth restriction).
• In late-onset FGR (after 32 weeks), UA Doppler is frequently normal, but CPR may be the only abnormal finding.
Normal physiological trend:
CPR is relatively stable across gestation but declines slightly with advancing gestational age. This is because both MCA PI and UA PI fall with gestation, but in late pregnancy the MCA PI falls faster than the UA PI in some fetuses, causing a natural decline in CPR near term. However, CPR should remain above 1.0 throughout gestation in a healthy fetus.
• 20–28 weeks: CPR typically 1.5–2.5
• 28–34 weeks: CPR typically 1.3–2.0
• 34–40 weeks: CPR typically 1.1–1.8
• At term (40 weeks): CPR may approach ~1.0–1.2 in normal fetuses, but should not fall below 1.0
Cut-off values:
There are two commonly used thresholds for defining abnormal CPR:
1. Fixed cut-off:
CPR < 1.0 is considered abnormal at any gestational age.
Simple and easy to remember, but less precise because it does not account for the natural gestational-age-related decline in CPR.
2. Centile-based cut-off (preferred):
CPR < 5th centile (or < 10th centile) for the gestational age.
More accurate — accounts for the gestational-age-related decline in CPR.
This is the recommended method by most guidelines (ISUOG, Delphi consensus).
3. Multiple of the Median (MoM):
CPR < 0.6765 MoM is considered abnormal.
Used in some research settings and reference charts.
CPR in the context of fetal deterioration:
In fetal growth restriction (FGR), the Doppler deterioration sequence typically follows this order:
1. CPR falls (brain-sparing begins — MCA dilates, UA PI may still be normal)
↓
2. MCA PI falls below 5th centile (cerebral vasodilation overt)
↓
3. UA PI rises above 95th centile (placental resistance rising)
↓
4. UA AEDF (absent end-diastolic flow)
↓
5. UA REDF (reversed end-diastolic flow)
↓
6. MCA PI normalises / rises (brain-sparing lost — decompensation; brain can no longer maintain vasodilation)
↓
7. Abnormal CTG / BPP (overt fetal distress)
Key point: CPR is the earliest Doppler marker to become abnormal in FGR — it changes before UA PI, MCA PI alone, or CTG.
Early-onset vs late-onset FGR:
• Early-onset FGR (<32 weeks): Placental insufficiency is severe. UA Doppler is usually abnormal (elevated PI, AEDF, REDF). CPR is low. MCA PI is low. The sequence is: CPR ↓ → UA PI ↑ → AEDF → REDF → MCA normalisation → CTG abnormal.
• Late-onset FGR (≥32 weeks): Placental insufficiency is milder and more subtle. UA Doppler is frequently normal. The only abnormal finding may be a low CPR (with or without low MCA PI). This makes CPR essential in late-onset FGR screening.
Measuring technique for CPR:
CPR requires two separate Doppler measurements — the MCA PI and the UA PI — and then their ratio is calculated.
Step 1: Measure UA PI
1. Identify a free-floating loop of umbilical cord.
2. Apply color Doppler and place PW sample volume (2–3 mm) over the artery.
3. Angle of insonation close to 0° (angle correction not needed for PI).
4. Capture ≥3 uniform waveforms during fetal rest.
5. Record the mean UA PI.
Step 2: Measure MCA PI
1. Obtain an axial transverse plane of the fetal head at the level of the thalami and cavum septum pellucidum.
2. Activate color Doppler to visualize the Circle of Willis.
3. Identify the middle cerebral artery — the largest lateral branch of the Circle of Willis, running anterolaterally from the internal carotid artery.
4. Place the PW sample volume (2–3 mm) over the proximal third of the MCA, within 2 mm of its origin from the internal carotid artery (avoiding the distal segments where resistance is higher).
5. Angle of insonation close to 0° — ideally insonate the portion of the MCA where the vessel runs toward or away from the probe. PI is angle-independent, but a good angle ensures a clean waveform.
6. Ensure the fetus is in a quiet resting state (no breathing, no movement). Ideally measure when the fetus is not actively moving.
7. Avoid measuring during fetal breathing — it alters venous return and affects cerebral flow.
8. Capture ≥3 uniform waveforms.
9. Record the mean MCA PI.
Step 3: Calculate CPR
CPR = MCA PI ÷ UA PI
Most ultrasound machines auto-calculate CPR when both PI values are entered.
Important technical notes for MCA:
• Do not apply angle correction for MCA PI — PI is a ratio and is angle-independent.
• Measure the proximal third of the MCA — distal segments have higher resistance and will falsely lower MCA PI (and thus falsely lower CPR).
• Do not press the probe hard on the maternal abdomen — excessive pressure can alter fetal intracranial pressure and falsely elevate MCA PI.
• Ensure the fetal head is not deeply engaged in the pelvis — a deeply engaged head can compress the MCA and falsely elevate PI.
• Fetal behavioural state matters — active sleep or wakefulness increases cerebral flow and lowers MCA PI. Measure during quiet sleep.
CPR interpretation guide:
• CPR > 1.0 (or > 5th centile): Normal — no brain-sparing. Blood flow distribution between brain and placenta is balanced.
• CPR < 1.0 (or < 5th centile): Abnormal — brain-sparing is present. The fetus is redistributing blood toward the brain due to hypoxia. Indicates fetal compromise.
• CPR < 1.0 with normal UA PI: Typical of late-onset FGR — the placenta is still functioning adequately to maintain UA flow, but the fetus is already experiencing subtle hypoxia and is brain-sparing.
• CPR < 1.0 with elevated UA PI: More advanced placental insufficiency — both brain-sparing and elevated placental resistance are present.
• CPR normalising after being low: May indicate loss of brain-sparing (decompensation) — the brain can no longer maintain vasodilation. This is a dangerous sign suggesting impending fetal demise.
CPR vs. individual UA and MCA Doppler:
• UA PI alone: Tells you about placental resistance, but cannot detect cerebral redistribution. May be normal in late-onset FGR.
• MCA PI alone: Tells you about cerebral resistance, but cannot tell you if the cause is hypoxia or a normal variant.
• CPR (MCA PI ÷ UA PI): Combines both — detects the redistribution between brain and placenta. More sensitive than either alone for detecting early fetal compromise.
• CPR is complementary — not a replacement for UA and MCA PI individually. Report all three.
Normal CPR Reference Values by Gestational Age
Clinical Decision Thresholds (CPR)
CPR Findings in Different Clinical Scenarios
CPR vs. other Doppler indices — summary comparison:
• UA PI: Measures placental resistance. Good for early-onset FGR. Insensitive for late-onset FGR.
• MCA PI: Measures cerebral resistance. Detects brain-sparing. Less specific when used alone.
• CPR (MCA PI ÷ UA PI): Combines both. Most sensitive marker for early fetal compromise. Detects brain-sparing before UA or MCA alone are abnormal. Essential for late-onset FGR.
• CPR is not a standalone test — always interpret with UA PI, MCA PI, and clinical context.
Key points to remember:
• CPR = MCA PI ÷ UA PI — a composite index comparing cerebral and placental resistance.
• Normal CPR is > 1.0 (or > 5th centile for GA).
• CPR < 1.0 (or < 5th centile) indicates brain-sparing — the fetus is shunting blood toward the brain due to hypoxia.
• CPR is the earliest Doppler marker to become abnormal in FGR — it changes before UA PI, MCA PI alone, or CTG.
• In late-onset FGR (≥32 weeks), UA Doppler is often normal and CPR may be the only abnormal finding.
• Always use gestational-age-specific centiles — CPR naturally declines near term, and a fixed cut-off of < 1.0 may miss or over-call cases at term.
• Loss of brain-sparing (CPR normalising after being low) is a danger sign — indicates fetal decompensation and impending demise.
• MCA PI must be measured in the proximal third of the vessel, at the level of the thalami, during fetal rest.
• Do not apply angle correction for PI measurements (both UA and MCA) — PI is angle-independent.
• Do not press hard on the maternal abdomen — can alter intracranial pressure and falsely elevate MCA PI.
• CPR is complementary to UA PI and MCA PI — report all three for a complete Doppler assessment.
• A low CPR is associated with adverse perinatal outcomes and long-term neurodevelopmental impairment, even when UA Doppler is normal.
Cerebroplacental Ratio (CPR) is the ratio of the pulsatility index (PI) of the middle cerebral artery (MCA) to the pulsatility index (PI) of the umbilical artery (UA). It is a composite Doppler index that compares cerebral vascular resistance with placental vascular resistance, providing a single number that reflects the balance of blood flow distribution between the fetal brain and the placenta.
Formula:
CPR = MCA PI ÷ UA PI
What it measures:
CPR assesses whether the fetus is preferentially shunting blood toward the brain at the expense of the placenta — a phenomenon known as "brain-sparing".
In a healthy fetus with a well-functioning placenta:
• The UA has low resistance (low UA PI) → blood flows easily to the placenta.
• The MCA has moderate-to-high resistance (higher MCA PI) → the brain does not need to dilate its vessels because oxygenation is adequate.
• Therefore CPR is > 1.0 (MCA PI > UA PI) — this is normal.
In a fetus with placental insufficiency and hypoxia:
• The UA has high resistance (high UA PI) → reduced placental flow.
• The MCA dilates to increase cerebral blood flow (low MCA PI) → the brain attempts to protect itself from hypoxia.
• Therefore CPR falls below 1.0 (MCA PI < UA PI) — this is brain-sparing and indicates fetal compromise.
Why CPR is important:
• CPR can become abnormal before either the UA PI or MCA PI alone crosses their respective 95th or 5th centile individually.
• It detects the redistribution phase of fetal adaptation to hypoxia — the brain dilates before the UA resistance becomes critically high.
• A low CPR is associated with adverse perinatal outcomes including IUGR, neonatal intensive care unit (NICU) admission, low Apgar scores, and long-term neurodevelopmental impairment.
• CPR is more sensitive than UA Doppler alone for detecting early fetal compromise, especially in late-onset FGR (fetal growth restriction).
• In late-onset FGR (after 32 weeks), UA Doppler is frequently normal, but CPR may be the only abnormal finding.
Normal physiological trend:
CPR is relatively stable across gestation but declines slightly with advancing gestational age. This is because both MCA PI and UA PI fall with gestation, but in late pregnancy the MCA PI falls faster than the UA PI in some fetuses, causing a natural decline in CPR near term. However, CPR should remain above 1.0 throughout gestation in a healthy fetus.
• 20–28 weeks: CPR typically 1.5–2.5
• 28–34 weeks: CPR typically 1.3–2.0
• 34–40 weeks: CPR typically 1.1–1.8
• At term (40 weeks): CPR may approach ~1.0–1.2 in normal fetuses, but should not fall below 1.0
Cut-off values:
There are two commonly used thresholds for defining abnormal CPR:
1. Fixed cut-off:
CPR < 1.0 is considered abnormal at any gestational age.
Simple and easy to remember, but less precise because it does not account for the natural gestational-age-related decline in CPR.
2. Centile-based cut-off (preferred):
CPR < 5th centile (or < 10th centile) for the gestational age.
More accurate — accounts for the gestational-age-related decline in CPR.
This is the recommended method by most guidelines (ISUOG, Delphi consensus).
3. Multiple of the Median (MoM):
CPR < 0.6765 MoM is considered abnormal.
Used in some research settings and reference charts.
CPR in the context of fetal deterioration:
In fetal growth restriction (FGR), the Doppler deterioration sequence typically follows this order:
1. CPR falls (brain-sparing begins — MCA dilates, UA PI may still be normal)
↓
2. MCA PI falls below 5th centile (cerebral vasodilation overt)
↓
3. UA PI rises above 95th centile (placental resistance rising)
↓
4. UA AEDF (absent end-diastolic flow)
↓
5. UA REDF (reversed end-diastolic flow)
↓
6. MCA PI normalises / rises (brain-sparing lost — decompensation; brain can no longer maintain vasodilation)
↓
7. Abnormal CTG / BPP (overt fetal distress)
Key point: CPR is the earliest Doppler marker to become abnormal in FGR — it changes before UA PI, MCA PI alone, or CTG.
Early-onset vs late-onset FGR:
• Early-onset FGR (<32 weeks): Placental insufficiency is severe. UA Doppler is usually abnormal (elevated PI, AEDF, REDF). CPR is low. MCA PI is low. The sequence is: CPR ↓ → UA PI ↑ → AEDF → REDF → MCA normalisation → CTG abnormal.
• Late-onset FGR (≥32 weeks): Placental insufficiency is milder and more subtle. UA Doppler is frequently normal. The only abnormal finding may be a low CPR (with or without low MCA PI). This makes CPR essential in late-onset FGR screening.
Measuring technique for CPR:
CPR requires two separate Doppler measurements — the MCA PI and the UA PI — and then their ratio is calculated.
Step 1: Measure UA PI
1. Identify a free-floating loop of umbilical cord.
2. Apply color Doppler and place PW sample volume (2–3 mm) over the artery.
3. Angle of insonation close to 0° (angle correction not needed for PI).
4. Capture ≥3 uniform waveforms during fetal rest.
5. Record the mean UA PI.
Step 2: Measure MCA PI
1. Obtain an axial transverse plane of the fetal head at the level of the thalami and cavum septum pellucidum.
2. Activate color Doppler to visualize the Circle of Willis.
3. Identify the middle cerebral artery — the largest lateral branch of the Circle of Willis, running anterolaterally from the internal carotid artery.
4. Place the PW sample volume (2–3 mm) over the proximal third of the MCA, within 2 mm of its origin from the internal carotid artery (avoiding the distal segments where resistance is higher).
5. Angle of insonation close to 0° — ideally insonate the portion of the MCA where the vessel runs toward or away from the probe. PI is angle-independent, but a good angle ensures a clean waveform.
6. Ensure the fetus is in a quiet resting state (no breathing, no movement). Ideally measure when the fetus is not actively moving.
7. Avoid measuring during fetal breathing — it alters venous return and affects cerebral flow.
8. Capture ≥3 uniform waveforms.
9. Record the mean MCA PI.
Step 3: Calculate CPR
CPR = MCA PI ÷ UA PI
Most ultrasound machines auto-calculate CPR when both PI values are entered.
Important technical notes for MCA:
• Do not apply angle correction for MCA PI — PI is a ratio and is angle-independent.
• Measure the proximal third of the MCA — distal segments have higher resistance and will falsely lower MCA PI (and thus falsely lower CPR).
• Do not press the probe hard on the maternal abdomen — excessive pressure can alter fetal intracranial pressure and falsely elevate MCA PI.
• Ensure the fetal head is not deeply engaged in the pelvis — a deeply engaged head can compress the MCA and falsely elevate PI.
• Fetal behavioural state matters — active sleep or wakefulness increases cerebral flow and lowers MCA PI. Measure during quiet sleep.
CPR interpretation guide:
• CPR > 1.0 (or > 5th centile): Normal — no brain-sparing. Blood flow distribution between brain and placenta is balanced.
• CPR < 1.0 (or < 5th centile): Abnormal — brain-sparing is present. The fetus is redistributing blood toward the brain due to hypoxia. Indicates fetal compromise.
• CPR < 1.0 with normal UA PI: Typical of late-onset FGR — the placenta is still functioning adequately to maintain UA flow, but the fetus is already experiencing subtle hypoxia and is brain-sparing.
• CPR < 1.0 with elevated UA PI: More advanced placental insufficiency — both brain-sparing and elevated placental resistance are present.
• CPR normalising after being low: May indicate loss of brain-sparing (decompensation) — the brain can no longer maintain vasodilation. This is a dangerous sign suggesting impending fetal demise.
CPR vs. individual UA and MCA Doppler:
• UA PI alone: Tells you about placental resistance, but cannot detect cerebral redistribution. May be normal in late-onset FGR.
• MCA PI alone: Tells you about cerebral resistance, but cannot tell you if the cause is hypoxia or a normal variant.
• CPR (MCA PI ÷ UA PI): Combines both — detects the redistribution between brain and placenta. More sensitive than either alone for detecting early fetal compromise.
• CPR is complementary — not a replacement for UA and MCA PI individually. Report all three.
Normal CPR Reference Values by Gestational Age
| GA (weeks) | CPR (5th–95th centile) | Mean CPR | Fixed cut-off | Interpretation |
|---|---|---|---|---|
| 20 | 1.40–2.50 | 1.95 | < 1.0 | High cerebral resistance; low placental resistance — normal |
| 22 | 1.35–2.40 | 1.88 | < 1.0 | — |
| 24 | 1.30–2.30 | 1.80 | < 1.0 | — |
| 26 | 1.25–2.20 | 1.73 | < 1.0 | — |
| 28 | 1.20–2.10 | 1.65 | < 1.0 | — |
| 30 | 1.15–2.00 | 1.58 | < 1.0 | — |
| 32 | 1.10–1.90 | 1.50 | < 1.0 | CPR beginning to decline — monitor in late FGR |
| 34 | 1.05–1.80 | 1.43 | < 1.0 | — |
| 36 | 1.00–1.70 | 1.35 | < 1.0 | Lower end of normal approaches 1.0 — use centiles, not fixed cut-off |
| 38 | 0.95–1.60 | 1.28 | < 1.0 | 5th centile may dip below 1.0 — centile-based assessment essential |
| 40 | 0.90–1.50 | 1.20 | < 1.0 | At term, CPR is naturally lower — always use GA-specific centiles |
| 41–42 | 0.85–1.45 | 1.15 | < 1.0 | Post-term CPR may fall further — interpret with caution |
Clinical Decision Thresholds (CPR)
| CPR Finding | Pathophysiology | UA PI | MCA PI | Clinical Action |
|---|---|---|---|---|
| CPR > 5th centile (normal) | No brain-sparing; normal flow distribution | Normal | Normal | Routine surveillance |
| CPR < 5th centile (abnormal) with normal UA PI | Early brain-sparing; typical of late-onset FGR | Normal | Low or normal-low | Serial Doppler; weekly monitoring; assess growth; consider delivery at 37–38 weeks |
| CPR < 5th centile with elevated UA PI | Established placental insufficiency + brain-sparing | Elevated | Low | Close monitoring; consider delivery from 34–37 weeks depending on severity |
| CPR < 1.0 (fixed cut-off) | Brain-sparing — MCA PI < UA PI | Variable | Low | Assess overall clinical picture; serial Doppler; plan delivery timing |
| CPR normalising after being low (MCA PI rising back) | Loss of brain-sparing — decompensation | Elevated | Rising / normalising | Danger sign — deliver immediately if viable |
| CPR < 0.6765 MoM | Research / centile-based abnormal | Variable | Variable | Interpret as per local protocol |
CPR Findings in Different Clinical Scenarios
| Scenario | UA PI | MCA PI | CPR | Interpretation |
|---|---|---|---|---|
| Normal fetus | Normal (low) | Normal (higher than UA) | > 1.0 (normal) | Healthy placenta; no brain-sparing |
| Early brain-sparing (hypoxia beginning) | Normal | Falling | Falling, may cross < 5th centile | Earliest sign of fetal compromise; UA still normal |
| Late-onset FGR (≥32 weeks) | Normal | Low | < 5th centile | Typical pattern — CPR is the only abnormal Doppler finding |
| Early-onset FGR with placental insufficiency | Elevated | Low | < 1.0 (very low) | Both placental resistance and brain-sparing are present |
| Advanced FGR — decompensation | Very high (AEDF/REDF) | Normalising / rising | May appear to "improve" | Loss of brain-sparing — brain can no longer compensate; impending demise |
| Post-term fetus | Normal or slightly high | Normal or slightly low | Borderline low (~1.0) | Interpret with GA-specific centiles — CPR naturally falls at term |
CPR vs. other Doppler indices — summary comparison:
• UA PI: Measures placental resistance. Good for early-onset FGR. Insensitive for late-onset FGR.
• MCA PI: Measures cerebral resistance. Detects brain-sparing. Less specific when used alone.
• CPR (MCA PI ÷ UA PI): Combines both. Most sensitive marker for early fetal compromise. Detects brain-sparing before UA or MCA alone are abnormal. Essential for late-onset FGR.
• CPR is not a standalone test — always interpret with UA PI, MCA PI, and clinical context.
Key points to remember:
• CPR = MCA PI ÷ UA PI — a composite index comparing cerebral and placental resistance.
• Normal CPR is > 1.0 (or > 5th centile for GA).
• CPR < 1.0 (or < 5th centile) indicates brain-sparing — the fetus is shunting blood toward the brain due to hypoxia.
• CPR is the earliest Doppler marker to become abnormal in FGR — it changes before UA PI, MCA PI alone, or CTG.
• In late-onset FGR (≥32 weeks), UA Doppler is often normal and CPR may be the only abnormal finding.
• Always use gestational-age-specific centiles — CPR naturally declines near term, and a fixed cut-off of < 1.0 may miss or over-call cases at term.
• Loss of brain-sparing (CPR normalising after being low) is a danger sign — indicates fetal decompensation and impending demise.
• MCA PI must be measured in the proximal third of the vessel, at the level of the thalami, during fetal rest.
• Do not apply angle correction for PI measurements (both UA and MCA) — PI is angle-independent.
• Do not press hard on the maternal abdomen — can alter intracranial pressure and falsely elevate MCA PI.
• CPR is complementary to UA PI and MCA PI — report all three for a complete Doppler assessment.
• A low CPR is associated with adverse perinatal outcomes and long-term neurodevelopmental impairment, even when UA Doppler is normal.
Cerebroplacental Ratio (CPR) Calculator
Fetal Doppler assessment — Middle Cerebral Artery & Umbilical Artery Pulsatility Index
CPR = MCA-PI ÷ UA-PI
A low ratio indicates "brain-sparing" redistribution, suggesting possible placental insufficiency.
A low ratio indicates "brain-sparing" redistribution, suggesting possible placental insufficiency.
Please enter valid positive values for both MCA-PI and UA-PI.
Cerebroplacental Ratio
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Interpretation thresholds used: CPR ≥ 1.0 normal; 0.6765–1.0 borderline / low-normal;
< 0.6765 abnormal (often approximated as < 5th centile / low MoM). Always evaluate against
gestational-age-specific reference ranges and overall clinical context.
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