Objectives: To assess risk factors of retinopathy of prematurity leading to delayed retinal vascularization in the first stage of ROP. Methods:A retrospective study (2000 -2016) was performed in 275 infants who were born at a gestational age of ≤ 32 weeks. Delayed vascular development was calculated as the difference in avascular area measured in terms of disc diameters between patients with risk factor and those with no risk factor. Results:Intrauterine fetal demise of one twin, apnea, exchange tranfusion and intubation days (≥ 10 days) lead to delayed retinal vascularization of 1.74, 1.58, 1.17, and 1.11 disc diameters and of 3.35, 3.04, 2.25 and 2.13 weeks respectively. Delayed retinal vascular development was less than 1 disc diameter in other factors such as HS-PDA, sepsis, degree of hyaline membrane ≥ III, cerebral hemorrhage, birth weight (every 100 g less), gestational age (every week less), postnatal weight gain (every 1 g less/day) and cerebral hemorrhage. However, since these factors are quantitative (birth weight, gestational age and postnatal weight gain), the greater the adverse effect, the greater the delay will be. Conclusions:Intrauterine fetal demise of one twin, apnea, exchange tranfusion and intubation days (≥ 10 days) lead to delayed retinal vascular development more than 1 disc diameter at 4 -6 weeks of postnatal life.