## Doppler Assessment in IUGR **Key Point:** The cerebroplacental ratio (CPR) is the most sensitive early marker of placental insufficiency and fetal compromise in IUGR, calculated as MCA PI / UA PI. ### Sensitivity Hierarchy in IUGR Detection | Doppler Index | Sensitivity | Timing of Abnormality | Clinical Use | |---|---|---|---| | **Cerebroplacental Ratio (CPR)** | **Highest (earliest)** | **First abnormality** | Early detection, brain-sparing effect | | Umbilical Artery PI | High | Early-moderate | Placental resistance | | Middle Cerebral Artery PI | Moderate | Moderate-late | Cerebral vasodilation (brain-sparing) | | Ductus Venosus PI | Low | Late | Severe compromise, imminent delivery | **High-Yield:** CPR < 1.0 (or < 5th centile) indicates abnormal brain-sparing response and is an independent predictor of adverse perinatal outcome, even when UA PI is still normal. ### Pathophysiology In early placental insufficiency: 1. Fetus responds with **brain-sparing effect** (cerebral vasodilation → ↓ MCA PI) 2. Placental resistance increases (↑ UA PI) 3. CPR drops disproportionately before individual indices become abnormal 4. This ratio captures the **redistribution of blood flow** before absolute Doppler cutoffs are breached **Clinical Pearl:** An abnormal CPR with normal UA PI is a red flag for impending fetal decompensation and warrants closer surveillance or delivery planning, even if traditional UA PI criteria are not yet met. **Mnemonic:** **CPR = Brain-Sparing Ratio** — when the cerebral artery dilates (PI ↓) while placental resistance rises (UA PI ↑), the ratio crashes earliest. 
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