The association of ROP with oxygen alone is not so clear. Transient hyperoxemia alone is not sufficient. Many other factors, such as extreme prematurity, apnea, sepsis, hyper- and hypocapnia, intraventricular hemorrhage, anemia, exchange transfusion, hypoxia, lactic acidosis, and possibly erythropoietin, which is angiogenic, have been implicated. Experimental studies have focused chiefly on the role of oxygen. Oxygen monitoring is an important part of the care of the premature infant, although extreme prematurity is known to be the significant risk factor. Recent data from Hellström et al have shown that postnatal weight gain also significantly affects the development of ROP.