Objective: To identify gestational age (GA) specific risk factors for severe ROP (sROP). Study design: Single-center cohort stratified by GA into <24 weeks, 24–26 weeks and ≥27 weeks. Results: 132/1106 (11.9%) developed sROP. Time to full feeds was the only risk factor [HR 1.003 (1.001–1.006), p = 0.04] for infants<24 weeks GA. For infants 24–26 weeks GA, a higher GA was protective [HR 0.66 (0.51–0.85), p < 0.01], whereas steroids for bronchopulmonary dysplasia (BPD) [HR 2.21 (1.28–3.26), p < 0.01], patent ductus arteriosus (PDA) ligation [HR 1.99 (1.25–3.11), p < 0.01] and use of nitric oxide [HR 1.96 (1.11–3.30), p = 0.01] increased the hazard of sROP. Increasing birthweight was protective [HR 0.70 (0.54–0.89), p < 0.01] in infants ≥27 weeks GA. Cumulative hazard of sROP reached 1.0 by fifteen weeks for <24 weeks GA, 0.4 by twenty weeks for 24–26 weeks GA, and 0.05 by twenty weeks after birth for ≥27 weeks GA. Conclusions: Risk factors, cumulative hazard, and time to sROP vary by GA.