Red cells

The relatively hypoxic intrauterine environment means that the newborn is polycythaemic by adult standards, a phenomenon that self-corrects during the first 3 months of life by which time the normal infant is anaemic relative to adults. Neonatal red cells are also macrocytic by adult standards, a feature that also disappears during the first 6 months as HbA replaces HbF.

• Neonatal red cells show much greater variation in shape than those from adults, particularly in premature babies—alarming microscopists more used to adult blood films.

• Occasional nucleated red cells are normal in the first 24-48h of life.

• Iron lack is common around 12 months of age due to increased demand from 4 red cell mass and (often) poor oral intake—cows' milk has virtually no iron content. The MCV falls to what would be abnormally low levels for adults as a reflection of this.

• In healthy premature neonates all these red cell differences may be exaggerated, with a nadir Hb at 2-3 months of 8-9g/dL in those with birth weight 1-1.5kg.

• Children have slightly lower Hb than adults until puberty.

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