Immnature retina is incomplete retinal vessel development, but not ROP. The eye goes through a normal vessel development with retinal vessels grow from the optic nerve head towards the retinal periphery. If they grow normally, they reach the nasal retinal peiphery (ora seratta) by 36 weeks post conceptional age and temporal periphery by 40 weeks post conceptional age.
This is a stage of no retinopathy of prematurity. These vessels can grow normally or later get stopped to develop ROP.