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Hematologic problems arise frequently in the newborn period.1 This section
is focused on disorders specific to the newborn period. For further
discussion of the developmental changes that occur in hematopoiesis,
see Chapter 429. Hematologic disorders, including congenital disorders,
are discussed in detail in Section 23: Disorders of the Blood.
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Nucleated Red Blood
Cells
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Nucleated red blood cells (nRBCs) are not common in older children
but can be found frequently in the peripheral blood smear of newborns.8 They
represent circulating erythrocyte precursors, or normoblasts, which
are normally found in the bone marrow. Healthy term newborns may
have some nRBCs in the peripheral blood. However, elevated nRBCs may
represent the result of pathologic processes such as relative in
utero hypoxia resulting in increased erythropoietin, which stimulates
increased production of red blood cells. Although many laboratories
report nRBCs in relation to the number of white blood cells (nRBCs/100
WBC), it is probably more informative to express this as an absolute
number per unit volume (ie, nRBCs/mm3). A value
of 1000 nRBCs/mm3 may be considered the higher
limit of normal for a normal term newborn. Preterm newborns typically
have higher nRBCs at birth.
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An increase in the number of circulating nRBCs can represent
conditions of increased erythropoiesis or stress-mediated release
of normoblasts from the bone marrow. Increased erythropoiesis may
occur in times of chronic hypoxia as in preeclampsia and placental
insufficiency. Infants born to smoking mothers have been found to
have increased nRBCs.9 Increased erythropoiesis may result
from blood loss from any cause, including hemolysis, which occurs
in the context of red blood cell isoimmunization. Elevation of nRBCs also
occurs in infants of diabetic mothers and in congenital infections.
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Hypoxia does not have to be chronic to observe nRBC elevation
at birth. Acute stress and subacute stress are also associated with
increased circulating nRBCs. Elevated nRBCs in this context may
serve as a marker for fetal asphyxia.10,11 Increased numbers
of nRBCs correlates with lower umbilical cord pH and therefore may
reflect degree and duration of asphyxia.12,13 Chronic asphyxia
is typically associated with higher nRBCs than with acute asphyxia
events.14
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The mean hemoglobin concentration in the term newborn ranges
from 14 to 20 g/100 mL.15 In the first hours after
birth, the hemoglobin concentration can rise due to a relative reduction
in plasma volume. However, the hemoglobin level soon begins to decline,
reaching a “physiologic nadir” at around 2 to
3 months of age. Preterm newborns have lower hemoglobin concentrations
at birth and a larger decrease after birth, reaching a nadir at
an earlier age, as early as 6 weeks for very-low-birth-weight infants.16 The
causes of anemia in the newborn period include increased blood loss
or decreased production of red blood cells.
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Blood loss may occur before, during, or after birth. Fetal-to-maternal ...