Birth weight and gestational age are the most important risk factors for development of severe ROP. Other factors that are associated with the presence of ROP include anemia, poor weight gain, blood transfusion, respiratory distress, breathing difficulties and the overall health of the infant. There is active research into the correlation of levels of growth factors in the blood and ROP. Close monitoring has decreased the impact of oxygen use as a risk factor for development of ROP. Light levels do not affect severity of ROP.