The blood vessels of the retina begin to develop three months after conception and complete their development at the time of normal birth. When an infant is born very prematurely, the infant's eye development will be disrupted.

In infants who develop retinopathy of prematurity (ROP), the vessels grow abnormally from the retina into the normally clear gel that fills the back of the eye. Here, without support, the vessels are fragile and often hemorrhage into the eye.

This is followed by scar tissue development which pulls the retina loose from the inner surface of the eye and draws it toward the center of the globe, producing a retinal detachment. This can reduce vision or, if severe, result in complete blindness.

Many premature infants develop transient and mild abnormal retinal blood vessel growth that converts to normal growth without treatment. Only approximately 1 out of 10 infants with early changes will progress to more severe retinal disease.

In the past, excess use of oxygen to treat premature babies stimulated abnormal vessel growth. Currently, oxygen can be easily and accurately monitored, and this is rare.

Today, the risk of developing ROP is proportional to the severity of prematurity. Typically all babies less than 32-34 weeks gestation are screened for the condition. However, only the smallest premature babies, no matter what their gestational age, have the highest risk.