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In Reply.—Dr Fikar quite correctly notes that the reticulocytosis observed in almost all newborns accounts for some of the macrocytosis at this age. Usually by 5 days of life, however, both term and preterm infants will experience a decline in reticulocyte count to a level the same as or lower than that in older infants and children. Thus, the continued decline in MCV cannot solely be a reflection of changes in reticulocyte count. Active erythropoiesis generally ceases until 6 to 12 weeks of age in term infants and until 4 to 8 weeks of age in preterm infants. Even on resumption of active erythropoiesis, extremely tiny babies (who really ought still to be in utero) do not seem to experience a rise in MCV as they begin to produce new RBCs. This is an enigma that defies explanation.
STOCKMAN JA. Mean Corpuscular Volume-Reply. Am J Dis Child. 1981;135(8):770. doi:10.1001/archpedi.1981.02130320078029