In general, ROP is a disease affecting the youngest, smallest and sickest infants. Typically, this includes infants born earlier than 32 weeks of gestation and under 1500 grams (about 3.3 pounds). The smaller and more premature the infant, the greater the risk.

ROP is a disorder of the blood vessels of the retina (the light sensitive part of the eye) that occurs in some premature babies. ROP is one of the leading causes of blindness in children.

The eye is a hollow sphere, and the retina the inner lining of the eye wall. It is the structure responsible for capturing visual information and sending it to the brain. There are multiple layers to the retina. The blood vessels which supply oxygen to the retina lie in the innermost layers.