Comparing Alternative Ranibizumab Dosages for Safety and Efficacy in Retinopathy of Prematurity

Inclusion Criteria: * Bilateral ROP in zone I (stage 1+, 2+, 3+/-, AP-ROP) or ROP in central (=posterior) zone II (stage 3+, AP-ROP). Zone I is defined as twice the distance from the optic disc to the fovea measured temporally, posterior zone II is defined as three times the distance from the optic disc to the fovea measured temporally. * Legal representatives or their designates willing and able to attend regular study visits with the study infant. * Written informed consent to participate in the study (signed by all patient's legal representatives). Exclusion Criteria: * Pediatric conditions rendering the infant ineligible to anti-VEGF treatment or to repeated blood draws as evaluated by a neonatal ICU specialist and a study ophthalmologist. * Congenital brain lesions significantly impairing optic nerve function. * Severe hydrocephalus with significantly increased intracranial pressure. * Advanced stages of ROP with partial or complete retinal detachment (ROP stage 4 and 5). * ROP involving only the peripheral retina (i.e. peripheral zone II or zone III). * Known hypersensitivity to the study drug or to drugs with similar chemical structures. * Contraindications for an intravitreal injection as listed in ranibizumab SmPC. * Systemic use of anti-VEGF therapeutics. * Use of other investigational drugs - excluding vitamins and minerals - at the time of enrollment, or within 30 days or 5 half-lives prior to enrollment, whichever is longer.