APLN and retinopathy of prematurity: Pearson’s linear correlation analysis using apelin-13 level as the dependent variable and VEGF level as the independent variable showed a negative correlation between apelin-13 and VEGF levels in infants with and without ROP (r = 0.946, P < 0.001 and r = 0.868, P < 0.001, respectively; Figs. 4 and 5).