ROP is described by its location in the eye (the zone), by the severity of the disease (the stage) and by the appearance of the retinal vessels (plus disease). The first stage of ROP is a demarcation line that separates normal from premature retina. Stage 2 is a ridge which has height and width. Stage 3 is the growth of fragile new abnormal blood vessels. As ROP progresses the blood vessels may engorge and become tortuous (plus disease).