Hypothesis: A "critical" number of daily arterial oxygen desaturations during the first two weeks of life is a key risk factor for severe ROP. We will: a) Further define the role of VEGF, IGF, MMPs, and ROS in ROP and correlate the levels with the number of arterial oxygen desaturations. b) Establish and identify whether increased serum VEGF in infants with severe ROP is the diffusible isoform VEGF121. This isoform is formed from VEGF proteolysis by plasmin and MMPs. MMPs also cleave Notch/Dll4, which acts as a regulator of VEGF signaling.