Which of the following Doppler parameters is the FIRST to become abnormal in intrauterine growth restriction?

Explanation

## Doppler Assessment in IUGR ### Sequence of Doppler Abnormalities **Key Point:** The **cerebroplacental ratio (CPR)** is the FIRST Doppler parameter to become abnormal in IUGR, as it captures subtle redistribution of fetal blood flow before either the umbilical artery PI or MCA PI individually crosses their respective abnormal thresholds. The progressive Doppler changes in IUGR follow a predictable sequence: | Doppler Parameter | Abnormality | Timing in IUGR | Clinical Significance | |---|---|---|---| | **Cerebroplacental ratio (CPR)** | ↓ CPR (<5th centile or <1.0) | **First to become abnormal** | Earliest sign of fetal redistribution | | **Umbilical artery PI** | ↑ PI, then absent/reversed diastolic flow (AREDV) | Second | Placental vascular resistance ↑ | | **Middle cerebral artery PI** | ↓ PI (brain sparing) | Concurrent with UA changes | Fetal cerebral vasodilation | | **Ductus venosus PI** | ↑ DV PI, then reversed flow | Late finding | Severe compromise, imminent delivery | ### Pathophysiology 1. **Early placental insufficiency** → subtle rise in UA PI and/or subtle fall in MCA PI → CPR falls below normal range **before** either individual parameter is overtly abnormal 2. **Established placental dysfunction** → UA PI clearly elevated, MCA PI clearly reduced (brain-sparing effect) 3. **Severe deterioration** → ductus venosus flow reversal indicates severe acidemia and impending fetal demise ### Why CPR is the Earliest Marker The CPR is calculated as **MCA PI ÷ UA PI** (normal >1.0 or >5th centile for gestational age). Because it is a ratio, it amplifies small simultaneous changes in both vessels that individually remain within "normal" limits. Multiple studies (Khalil et al., ISUOG guidelines 2020; Baschat & Gembruch, Ultrasound Obstet Gynecol) confirm that CPR becomes abnormal earlier than isolated UA PI or MCA PI, and has superior predictive value for adverse perinatal outcomes — particularly in late-onset IUGR where UA PI may remain normal throughout. **High-Yield:** In **late-onset IUGR**, UA PI is often normal; CPR is the key Doppler marker. In **early-onset IUGR**, UA PI abnormality is prominent, but CPR still precedes overt UA PI elevation. **Clinical Pearl (ISUOG 2020):** CPR <5th centile or <1.0 is an indication for increased surveillance even when UA PI and MCA PI are individually within normal limits — underscoring that CPR is the most sensitive early Doppler indicator of fetal compromise in IUGR.