The purpose of this study is to determine whether increased transferrin saturation in plasma (that reflects iron overload and/or low transferrin) is an independent risk factor for ROP development and severity. Preterm infants born at \<31 week's post-menstrual age (PMA) or ≤1250g of birth weight will be included. Iron parameters in plasma will be measured during the first month of life. Retinopathy of prematurity (ROP) will be screened as currently recommended. The relationship between plasma iron parameters and ROP development and/or severity will be established.