The retina is the inner lining on the inside back part of the eye. A healthy retina is required for good vision. (If the eye were a camera, the retina would be the film – the part that captures the image.)

The blood vessels that supply the retina with Oxygen and nutrients are not completely developed or mature until around 40 weeks gestation, the age of a full term baby. But when a baby is born early the blood vessels are immature and need to continue to grow. About 4 to 8 weeks after birth the vessels may start to grow abnormally because the retina produces abnormal amounts of various factors which encourage rapid blood vessel growth. 

When this happens, the new vessel growth is too fragile. It may leak fluid, bleed, and cause scarring. This is called ROP.

Retinopathy Prematurity Examination