Blood volume starts to expand before the fourth week of pregnancy and continues to increase until the middle of the third trimester, when it is about 40 to 50% above the prepregnant level (Fig. 5). Although the amount of added volume varies widely from woman to woman, the increase is consistent for each woman in each of her pregnancies and is greater in twin than in single pregnancies. Both plasma and red cell volumes increase, but the plasma volume expands by 50%, while the red cell mass increases by only 20 to 30%. Consequently, the hematocrit declines from about 45% to about 35%, producing the so-called anemia of pregnancy. It is not known why expansion of the red cell mass fails to keep pace with the increase in plasma volume. Erythropoietic capacity of bone marrow is not the limiting factor. Further enhancement of red blood cell production is seen after hemor-rhagic injury. Similarly, red blood cell production increases further in pregnant women in response to the decrease in oxygen tension (PO2) encountered with a change in residence from sea level to high altitudes. Pregnancy thus does not interfere with the operation of basic regulatory mechanisms that govern erythropoiesis. Concentrations of erythropoietin (see Chapter 23) in blood plasma increase only modestly during pregnancy.
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