During pregnancy, the change in hematocrit is due to which of the following?

During pregnancy, the change in hematocrit is due to which of the following?

• A. Increase RBC mass
• B. Increase plasma volume in excess of increase in red blood cells
• C. Decrease plasma volume
• D. Increase white blood cell mass

Answer: (B)

During pregnancy, plasma volume expands more than red blood cell mass, so the hematocrit—the proportion of blood that is red cells—decreases because the blood is diluted by the greater rise in plasma. This dilutional effect is why there is a physiologic fall in hematocrit even though red cell production is increasing. An increase in red cell mass would raise hematocrit, a decrease in plasma volume would concentrate the blood and raise hematocrit, and white blood cell mass does not significantly change hematocrit. The best description is that plasma volume increases in excess of the red blood cell increase.

During pregnancy, plasma volume expands more than red blood cell mass, so the hematocrit—the proportion of blood that is red cells—decreases because the blood is diluted by the greater rise in plasma. This dilutional effect is why there is a physiologic fall in hematocrit even though red cell production is increasing. An increase in red cell mass would raise hematocrit, a decrease in plasma volume would concentrate the blood and raise hematocrit, and white blood cell mass does not significantly change hematocrit. The best description is that plasma volume increases in excess of the red blood cell increase.