Anemia is the reduction in red blood cell (RBC) number, hematocrit, or hemoglobin concentration to a value >2 SDs below the age-specific mean. Anemia in infancy, which may result from increased erythrocyte loss or inadequate RBC production, raises unique considerations.

Understanding the development of the hematopoietic system may be helpful in the evaluation of neonates with anemia. Erythropoiesis begins in the yolk sac at 2 weeks’ gestation, generating cells that express embryonic hemoglobin. At 6 weeks’ gestation, the liver becomes the predominant site of RBC production, and the cells produced primarily express fetal hemoglobin. Not until 6 months’ gestation does the bone marrow become the major site of hematopoiesis. Throughout fetal life, erythrocytes decrease in size and increase in number: hematocrit increases from 30% to 40% during the second trimester to 50% to 63% at term. In late gestation and after birth, RBCs gradually switch from the production of fetal...

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