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Pregnancy induces physiological changes that often confuse the diagnosis of hematological disorders and assessment of their treatment. This is especially true for anemia. A number of pregnancy-induced hematological changes are discussed in detail in Chapter 5, Hematological Changes. One of the most significant changes is blood volume expansion with a disproportionate plasma volume increase, resulting in a normally decreased hematocrit.

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Pregnant women are susceptible to hematological abnormalities that may affect any woman of childbearing age. These include chronic disorders such as hereditary anemias, immunological thrombocytopenia, and malignancies, including leukemias and lymphomas. Other disorders arise during pregnancy because of pregnancy-induced demands, two examples being iron-deficiency and megaloblastic anemias. Pregnancy may also unmask underlying hematological disorders such as compensated hemolytic anemias caused by hemoglobinopathies or red cell membrane defects. Finally, any hematological disease may first arise during pregnancy, such as autoimmune hemolysis or aplastic anemia.

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Extensive hematological measurements have been made in healthy nonpregnant women. Concentrations of many cellular elements that are normal during pregnancy are listed in the Appendix. As shown in Table 51-1, anemia is defined as hemoglobin concentration less than 12 g/dL in nonpregnant women and less than 10 g/dL during pregnancy or the puerperium. The Centers for Disease Control and Prevention (1998) defined anemia in iron-supplemented pregnant women using a cutoff of the 5th percentile—11 g/dL in the first and third trimesters, and 10.5 g/dL in the second trimester.

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Table 51-1. Hemoglobin Concentrations in 85 Healthy Women with Proven Iron Stores
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The modest fall in hemoglobin levels during pregnancy is caused by a relatively greater expansion of plasma volume compared with the increase in red cell volume (Fig. 51-1). The disproportion between the rates at which plasma and erythrocytes are added to the maternal circulation is greatest during the second trimester. Late in pregnancy, plasma expansion essentially ceases, while hemoglobin mass continues to increase.

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Figure 51-1
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Mean hemoglobin concentrations (black line) and 5th and 95th percentiles (blue lines) for healthy pregnant women taking iron supplements. (Data from Centers for Disease Control and Prevention, 1989.)

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After delivery, the hemoglobin level fluctuates and then rises to and usually exceeds the nonpregnant level. The rate and magnitude of increase early in the puerperium result from the amount of hemoglobin added during pregnancy and the amount of blood loss at delivery modified by normally decreasing plasma volume postpartum.

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