A pregnant woman at 28 weeks gestation presents with physiological anemia. Her Hb is 10.2 g/dL, hematocrit 31%. The physiological mechanism responsible is:

• A Plasma volume expansion (40–50%) exceeding RBC mass expansion (20–30%), causing dilutional pseudoanemia despite increased total RBC mass ✓
• B Reduced erythropoietin production due to renal hemodilution
• C Fetal competition for iron stores reducing maternal erythropoiesis
• D Progesterone-mediated suppression of bone marrow erythropoiesis

Explanation

In normal pregnancy, plasma volume increases by 40–50% (beginning at 6 weeks, peaking at 28–34 weeks), while RBC mass increases only 20–30%. The disproportionate plasma expansion relative to RBC mass produces hemodilution—a physiological 'anemia' with reduced Hb concentration and hematocrit despite an absolute increase in total circulating hemoglobin and RBC mass. This dilutional anemia is not pathological; it reduces blood viscosity, facilitating placental perfusion. Erythropoietin actually increases in pregnancy, not decreases. Iron requirements do increase (needed for expanded RBC mass and fetal/placental demands), but iron deficiency would be superimposed true anemia. Progesterone does not suppress erythropoiesis.

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

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