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I. DEFINITION

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Anemia developing during the neonatal period (0–28 days of life) in infants of >34 weeks' gestational age is indicated by a central venous hemoglobin <13 g/dL or a capillary hemoglobin <14.5 g/dL.

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II. INCIDENCE

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Anemia is the most common hematologic abnormality in the newborn. Specific incidence depends on the cause of the anemia.

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III. PATHOPHYSIOLOGY

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  1. Normal physiology. At birth, normal values for the central venous hemoglobin in infants of >34 weeks' gestational age are 14–20 g/dL, with an average value of 17 g/dL. Reticulocyte count in the cord blood of infants ranges from 3–7%. The average mean corpuscular volume of red blood cells (RBCs) is 107 fL. Premature infants have slightly lower hemoglobin and higher mean corpuscular volume and reticulocyte counts. In healthy term infants, hemoglobin values remain unchanged until the third week of life and then decline, reaching a nadir of 11 g/dL at 8–12 weeks. This is known as the “physiologic anemia of infancy.” In preterm infants, this decline is more profound, reaching a nadir of 7–9 g/dL at 4–8 weeks. This exaggerated physiologic anemia of prematurity is related to a combination of decreased RBC mass at birth, increased iatrogenic losses from laboratory blood sampling, shorter RBC life span, inadequate erythropoietin production, and rapid body growth. In the absence of clinical complications associated with prematurity, infants remain asymptomatic during this process.

  2. Etiologies of anemia. Anemia in the newborn infant results from 1 of 3 processes: loss of RBCs, or hemorrhagic anemia, the most common cause; increased destruction of RBCs, or hemolytic anemia; or underproduction of RBCs, or hypoplastic anemia.

    1. Hemorrhagic anemia

      1. Antepartum period (1 in 1000 live births)

        1. Loss of placental integrity. Abruptio placentae, placenta previa, or traumatic amniocentesis (acute or chronic) may result in loss of placental integrity.

        2. Anomalies of the umbilical cord or placental vessels. Velamentous insertion of the umbilical cord occurs in 10% of twin gestations and almost all gestations with >3 fetuses. Communicating vessels (vasa praevia), umbilical cord hematoma (1 in 5500 deliveries), or entanglement of the cord by the fetus may also cause hemorrhagic anemia.

        3. Twin-twin transfusion. Observed only in monozygotic multiple births. Monozygotic (MZ) twin pregnancies account for ∼30% of spontaneously conceived twins. The occurrence of MZ twins is 0.4–0.45% of nonstimulated in vivo conceptions. The incidence of monochorionic twins is rising, owing to the increase in the use of assisted reproductive technology (ART). The use of ART has been associated with a 2- to 12-fold increase in the conception of MZ twins. In the presence of a monochorionic placenta, 13–33% of twin pregnancies are associated with twin-twin transfusion. The difference in hemoglobin concentration between twins is >5 g/dL. The anemic donor twin may develop congestive heart disease, whereas the recipient plethoric twin may manifest signs of the hyperviscosity syndrome. In utero laser photocoagulation, which interrupts the vascular connections on the chorionic plate, has improved the low survival rate for twin-twin transfusion diagnosed before 26 weeks' ...

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