Physiologic anemia of pregnancy results from which physiologic change?

• A. Red cell mass expanding more than plasma volume
• B. Iron deficiency
• C. Hemolysis
• D. Plasma volume expanding more than red cell mass

Answer: (D)

The main idea is dilutional anemia: during pregnancy, plasma volume expands more than red cell mass, so the red cells become diluted in a larger volume of blood.

Plasma volume increases significantly—roughly about half of its typical amount by the later stages of pregnancy—while red cell mass rises much more modestly, on the order of a quarter to a third. Because hematocrit (the proportion of red cells in blood) depends on the balance between red cell mass and plasma volume, this disproportionate expansion lowers Hb concentration and hematocrit without reflecting a true iron shortage or increased destruction of red cells. That’s why this is considered a normal physiologic adaptation.

Iron deficiency would produce anemia due to insufficient iron, often with microcytosis and low ferritin, not just dilution. Hemolysis would involve accelerated red cell destruction and signs like elevated bilirubin and reticulocytosis. If red cell mass expanded more than plasma, hematocrit would rise, not fall.