Whole blood viscosity is determined by hematocrit, plasma viscosity, red cell deformation, and red cell aggregation. Hyperviscosity is thought to be prevalent in neonates due to polycythemia and diminished red cell deformation. […] Blood from low birth weight and full term neonates is significantly hypoviscous when compared to that of adults. This hypoviscosity is related to low plasma viscosity due to lower plasma proteins and fibrinogen in neonates.
Source: wiktionary