When preterm infants are deprived of their natural intrauterine environment they lose access to important factors, normally found in utero, such as proteins, growth factors, and cytokines. It has been demonstrated that insulin-like growth factor-1 (IGF-1) is one such factor. In utero these biological factors are introduced to the fetus via placental absorption or ingestion from amniotic fluid. Deprivation of such factors is likely to cause inhibition or improper stimulation of important pathways, which in the case of the eye may cause abnormal retinal vascular growth, the hallmark of retinopathy of prematurity (ROP).