 The primary mechanism of AOP is a decrease in erythropoietin (EPO), a red blood cell growth factor.Preterm infants are often anemic and typically experience heavy blood losses from frequent laboratory testing in the first few weeks of life.Although their anemia is multifactorial, repeated blood sampling and reduced erythropoiesis with extremely low serum levels of erythropoietin (EPO) are major causative factors.