Children who had a top quartile concentration of IL-6 on both days were at increased risk of ROP when the concentration of the neurotrophic protein (BDNF, IGF-1, IGFBP-1, VEGF, VEGFR-1 and 2, ANG-1, or PIGF) was not in the top quartile on both days. Here, PIGF is linked to retinopathy of prematurity.