The disease typically progresses in four stages: The disease typically progresses in four stages: Mild non-proliferative retinopathy causes small balloon-like swelling of the retinal blood vessels to occur. Severe non-proliferative retinopathy blocks more blood vessels, depriving areas of the retina of blood supply. Moderate non-proliferative retinopathy blocks the blood vessels that nourish the retina. Proliferative retinopathy causes the body to develop new, but abnormal, blood vessels with thin walls.