A 28-year-old primigravida at 32 weeks of gestation presents with a fundal height 4 cm less than expected for gestational age. Doppler ultrasound shows normal umbilical artery pulsatility index but reduced cerebroplacental ratio. Which is the most common cause of intrauterine growth restriction in this clinical scenario?

Explanation

## Most Common Cause of IUGR **Key Point:** Chronic placental insufficiency is the single most common cause of intrauterine growth restriction (IUGR), accounting for approximately 40–50% of all cases. It represents the final common pathway through which multiple maternal, placental, and fetal factors impair fetal growth. ### Why Chronic Placental Insufficiency is the Answer The clinical scenario — fundal height 4 cm less than expected, **normal umbilical artery pulsatility index**, and **reduced cerebroplacental ratio (CPR)** — is the textbook presentation of **early/compensated chronic placental insufficiency**: 1. **Reduced CPR with normal UA PI** = fetal brain-sparing response (cerebral vasodilation) before umbilical artery resistance rises — the earliest Doppler sign of placental compromise 2. **Late-onset IUGR at 32 weeks** — characteristic of uteroplacental insufficiency rather than structural anomalies or infections (which typically manifest earlier) 3. **Chronic placental insufficiency** encompasses impaired trophoblast invasion, spiral artery remodeling failure, and reduced intervillous perfusion — regardless of the underlying trigger ### Causes of IUGR and Their Relative Frequency | Cause | Prevalence in IUGR | Typical Doppler Pattern | |-------|-------------------|------------------------| | **Chronic placental insufficiency** | ~40–50% | Reduced CPR → elevated UA PI → absent/reversed EDF | | Maternal hypertension/preeclampsia | Subset of placental insufficiency | Same as above | | Congenital fetal anomalies | 15–20% | Usually normal Doppler | | Intrauterine infections (TORCH) | 5–10% | Usually normal Doppler | **High-Yield:** Maternal hypertension and preeclampsia are important *causes* of chronic placental insufficiency, but "chronic placental insufficiency" is the broader, most common mechanistic category. The question asks for the most common cause of IUGR in this clinical scenario — the Doppler findings (reduced CPR, normal UA PI) point directly to placental insufficiency as the mechanism, not specifically to hypertension (which is not mentioned in the stem). **Clinical Pearl:** The cerebroplacental ratio (MCA PI ÷ UA PI) < 1.0 or below the 5th centile for gestational age is the earliest and most sensitive Doppler marker of fetal compromise in placental insufficiency. It precedes UA PI elevation and indicates fetal cerebral vasodilation ("brain-sparing"), as described in this case. ### Management of IUGR Due to Placental Insufficiency - **Serial Doppler surveillance** — Weekly or biweekly UA and MCA Doppler - **Corticosteroids** — Betamethasone if delivery anticipated before 34 weeks - **Delivery timing** — At 37 weeks if stable; earlier if Doppler deteriorates (absent/reversed EDF) or biophysical profile is compromised [cite: Williams Obstetrics 26e Ch 40; RCOG Green-top Guideline No. 31 — Small-for-Gestational-Age Fetus]