Since twin pairs have equal gestational age, they investigated the specific effect that birth weight may have on the development of ROP and showed that birth weight plays an important role in any stage of ROP.[23] Insulin-like growth factor I (IGF-I) is an important somatic growth factor for fetal and neonates' growth and development that is correlated with birth weight.[24,25] Early levels of low IGF-I are associated with slower-than-expected weight gain and more severe ROP.[26] Therefore, birth weight is a more important factor than gestational age as it pertains to the development of RO. This evidence concerns the gene IGF1 and retinopathy of prematurity.