A growth restricted fetus shows a head circumference at the 50th percentile while the abdominal circumference falls below the 3rd percentile, with reduced middle cerebral artery pulsatility and elevated umbilical artery Doppler indices. This pattern of growth restriction is most typically caused by which mechanism?

• A Early onset chromosomal anomaly acting throughout pregnancy
• B Congenital rubella infection in the first trimester
• C Primary skeletal dysplasia limiting abdominal organ growth
• D Late onset uteroplacental insufficiency triggering circulatory redistribution to the brain ✓

Explanation

Asymmetric growth restriction arises when placental insufficiency develops in the third trimester. Fetal hypoxaemia triggers the brain sparing reflex, redistributing cardiac output preferentially to the brain, so head growth is relatively preserved while liver glycogen stores fall and abdominal circumference lags. Early insults such as aneuploidy or first trimester infection restrict all biometric parameters equally, producing symmetric restriction with a proportionally small head.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.