Primary marker of placental resistance. Decreased or reversed end-diastolic flow (AEDF/REDF) indicates severe failure.
Middle Cerebral (MCA)
Reflects fetal compensation for hypoxia. Decreased PI indicates "Head Sparing" shunting.
Cerebroplacental (CPR)
MCA PI / UA PI Ratio. Index < 1st centile is the most sensitive predictor of adverse outcomes.
Critical Red Flags (UA)
AEDF
Absent End Diastolic Flow
REDF
Reversed End Diastolic Flow
Guidelines & Evidence
Clinical Details
Section 1
When to Use
Primary Clinical Uses
Fetal surveillance in pregnancies complicated by Fetal Growth Restriction (FGR)
Monitoring maternal-fetal hemodynamics in early-onset and late-onset preeclampsia
Screening and diagnosing fetal anemia (via MCA Peak Systolic Velocity)
Guiding the optimal timing of delivery in chronic placental insufficiency
When NOT to Use
Routine Doppler screening in universally low-risk, uncomplicated pregnancies is not recommended as it does not improve perinatal outcomes and may precipitate unnecessary anxiety and iatrogenic interventions.
Section 2
Formula & Logic
The Vasculature
Umbilical Artery (UA)
Reflects placental resistance. A normal placenta operates as a low-resistance sink. Abnormal resistance increases the Pulsatility Index (PI) and ultimately halts or reverses diastolic flow.
Middle Cerebral Artery (MCA)
Reflects fetal cardiovascular adaptation. Under hypoxic stress, the fetus dilates cerebral vessels ("brain-sparing effect") to maintain CNS oxygenation, which appears as an abnormally low PI on Doppler.
Cerebroplacental Ratio (CPR)
CPR = (MCA-PI) / (UA-PI)
CPR Interpretation
The CPR is an integrated index. A CPR < 1 (or < 5th percentile for gestational age) is highly abnormal. It essentially flags fetuses that have simultaneously high placental resistance AND cerebral vasodilation (the brain-sparing reflex in full swing).
Section 3
Pearls/Pitfalls
Key Strengths
Identifies placental insufficiency and fetal redistribution of blood flow highly accurately, significantly earlier than overt signs of decompensation (like late decelerations on an NST)
Absent or Reversed End-Diastolic Flow (AEDF/REDF) in the umbilical artery are definitive, objective, universally agreed-upon triggers for heavy clinical intervention
Known Technical Limitations
Highly angle-dependent: Accuracy evaporates if the angle of insonation exceeds 30° mathematically (close to 0° is strictly required for ideal MCA velocities)
Measurements must exclusively be taken during fetal apnea and absent gross motor movements, as fetal breathing dramatically distorts the waveform
Cannot be interpreted independently of customized, gestational-age-specific nomograms
Section 4
Next Steps
Umbilical Artery PI > 95th Percentile
01
Identifies established Fetal Growth Restriction (FGR)
02
Increase surveillance frequency (e.g., weekly Dopplers, BPP)
03
Administer maternal corticosteroids for fetal lung maturity if delivery is anticipated preterm
UA Absent/Reversed End-Diastolic Flow (AEDF/REDF)
01
Signifies critical placental failure
02
Admit to hospital for continuous or extremely frequent fetal monitoring
03
Plan for emergent delivery depending on exact gestational age, steroid coverage, and fetal biophysical profile limits
MCA-PI < 5th Percentile (Brain-Sparing)
01
Signals chronic hypoxia and fetal auto-redistribution
02
In late-onset FGR (typically near term), an isolated low MCA-PI strongly prompts delivery even if the umbilical artery is technically normal
Section 5
Evidence Appraisal
Guidelines
Fetal Medicine Foundation protocols established the paradigm of Doppler integration. Modern SMFM guidelines strongly endorse relying upon rigorous UA Doppler over arbitrary biometry cutoffs to sequence delivery in FGR.
Section 6
Literature
Physics Application
Relying upon the Doppler Effect (first described by Christian Doppler in 1842), ultrasound waves bouncing off moving erythrocytes in fetal vasculature change frequency proportionally to blood velocity, allowing modern computers to build precise mathematical resistive indices without invading the uterus.
