Levels of sphingosine-1-phosphate (S1P) in infant serum dichotomously grouped by final retinopathy of prematurity (ROP) stage, No/moderate ROP (stages 0–2, n = 28) and Severe ROP (stage 3 and/or Type 1 ROP, n = 19). A) Box plots with serum S1P level by time point. PND, postnatal day; PMW, postmenstrual week. B) Scatter plot of serum S1P levels by postnatal day of sampling and estimates from the mixed model for repeated measures adjusted for GA at birth and weight SDS. 95% CI are presented for the curves. Graph area highlighted in yellow represents time points where curves differ significantly after adjustment for multiplicity. The relative difference between Severe ROP and No/moderate ROP in the analysis adjusted for GA at birth and weight SDS change from birth over the study period (Scheffe-Holm adjusted p-values and 95% CIs) was: at 1 day, 0.86 (0.58–1.28) p = 0.77; at 14 days, 0.75 (0.53–1.07) p = 0.16; at 28 days, 0.66 (0.47–0.92) p = 0.0090; at 54 days, 0.54 (0.37–0.80) p = 0.0005; at 86 days, 0.58 (0.39–0.86) p = 0.0029; Overall test severe ROP vs No/moderate ROP p = 0.0002; Interaction TIME*GROUP p = 0.0013.