Polycythemia, defined as a central venous hematocrit (Hct) level of greater than 65%, is a relatively common disorder. The primary concern with polycythemia is related to hyperviscosity and its associated complications.  Blood viscosity increases exponentially as the Hct level rises above 42%. This associated hyperviscosity is thought to contribute to the symptom complex observed in approximately one half of infants with polycythemia. However, only 47% of infants with polycythemia have hyperviscosity, and only 24% of infants with hyperviscosity have a diagnosis of polycythemia. 
As the central Hct level increases, viscosity increases. The arterial oxygen content also increases. Changes in blood flow are observed in some organs; this is due to changes in viscosity or changes in arterial oxygen content. The change in blood flow may influence oxygenation and may influence the delivery of substances to organs that are dependent on plasma flow, such as glucose.
Many factors determine blood viscosity. The primary factor in the newborn is the Hct. As such, viscosity increases as Hct level rises. Other factors that uniquely contribute to blood viscosity in the neonate include increased RBC volume and decreased deformability of the fetal erythrocyte. Plasma proteins, platelets, WBCs, and endothelial factors also contribute to viscosity; however, they are not clinically significant in the newborn.
Polycythemia occurs in 1-5%% of neonates.  It is more common in infants who are small for their gestational age (SGA) and in infants who are large for their gestational age (LGA). However, most infants with polycythemia are of appropriate size or weight for their gestational age (AGA). Infants of mothers with diabetes have an incidence of more than 40%, and those born to mothers with gestational diabetes have an incidence of more than 30%. Polycythemia is also common in infants who have experienced delayed clamping of the umbilical cord. Hyperviscosity occurs in 6.7% of infants.
The central venous Hct level peaks 6-12 hours after birth and then declines until the infant is aged 24 hours, at which time it equals the Hct level in cord blood. Fewer than 40% of infants with a Hct level greater than 64% at 2 hours still have a high value at 12 hours or later.