The two biggest risk factors for developing ROP are premature birth (most common in babies born more than 12 weeks early) and a low birth weight (most common in 11% - 56% of preterm infants with low birth weight). A baby's eyes develop the most during the last 3 months of pregnancy in the womb. When a baby is born early, the development of the retina is incomplete and there is a risk that the blood vessels which supply oxygen to the eye may not grow normally, which can lead to ROP. 
Other than premature birth and low birth weight, many other factors can place these babies at higher risk of ROP. This includes prolonged use of oxygen (for their lungs), sepsis (blood infection), blood transfusion, jaundice, low weight gain, low platelets, anaemia (low haemoglobin), intraventricular haemorrhage (bleeding in the brain), etc.