Up to now, only data about early and single coagulation screening exist in extreme low birth weight infants (ELBW) infants. In neonatal practice, coagulation abnormality in preterm babies is primarily investigated by measuring prothrombin time (PT). In fact, FVII activity, which is an important determinant of PT, is strongly associated with bleeding risk. Thus, a method to measure PT with small volume samples (10μL) provides the possibility for serial monitoring even in ELBW infants (CoaguChek®XS, Roche Diagnostics, Vienna, Austria)). Substitution of fesh frozen plasma seemed beneficial in ELBW infants and first trials with rFVII revealed promising results in this patient population. Thus, coagulation monitoring might lead to early and adequate therapy and therefore to a better outcome. The investigators hypothesize that ELBW infants (<1000g birth weight) with primary severe prolongation of prothrombin time or development of severe prothrombin prolongation during sequential monitoring may have more frequent and more severe bleeding incidents (Intraventricular haemorrhage and pulmonary haemorrhage). The investigators aim to explore whether monitoring of PT can predict bleeding events in ELBW children.