Anemia and polycythemia in the newborn.

摘要

Red blood cell (RBC) mass in the newborn is highly variable. As an infant makes the transition from the intrauterine to the extrauterine environment, a change occurs in both the mass and the composition of RBCs. The range of normal, while clearly defined, is wide because of the many fluctuating variables involved in the physiology of the peripartum period.In utero, fetal hemoglobin predominates. When compared with adult hemoglobin, fetal hemoglobin has enhanced oxygen-binding capacity, a characteristic that allows sufficient ox- ygen transfer to the fetus in the absence of gas exchange with the external environment. Even with fetal hemoglobin's increased affinity for oxygen, the intrauterine environment is relatively hypoxic. As a result, the hemoglobin level in a near-term fetus or term infant is relatively high. The normal hemoglobin concentration for a term newborn is 19.3?.2 g/dL (193?20 g/L), with a hematocrit of 61?o?.4?o (0.61 ?.074), values that continue to rise until they reach a maximum at about 2 hours after birth.