Placental function undergoes marked physiological changes in the third trimester. Which statement about uteroplacental blood flow BEST describes why placental insufficiency causes symmetric IUGR when it occurs early vs. asymmetric IUGR when it occurs late?

• A Early insufficiency impairs both cell number (hyperplastic phase) and size (hypertrophic phase); late insufficiency mainly limits cell hypertrophy while sparing brain growth via brain-sparing reflex
• B Early insufficiency impairs only cell size in already-differentiated organs; late insufficiency reduces overall cell number
• C Late insufficiency predominantly impairs liver size while the brain-sparing reflex maintains head growth via redistributed cardiac output ✓
• D Both symmetric and asymmetric IUGR result from chronic uteroplacental insufficiency but are distinguished only by gestational age at diagnosis

Explanation

Asymmetric IUGR (late placental insufficiency) results from preferential redistribution of fetal cardiac output to the brain and adrenals ('brain-sparing reflex') at the expense of visceral organs, particularly the liver and abdominal organs. Thus head circumference is relatively preserved while abdominal circumference (liver size) is reduced — giving the asymmetric pattern. In early-onset (symmetric) IUGR, all organ systems including the brain are affected equally during the hyperplastic growth phase. Option A is partially correct conceptually but option C best captures the specific brain-sparing physiology that explains asymmetry.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.