Neonates are unique due to maturational differences in their coagulation systems. During the first few months of life, distinct differences exist between the coagulation system of a neonate and that of an adult including differences between the concentration of coagulation proteins, the ability to generate thrombin and the ability to inhibit thrombin once it is formed (1). One important coagulant protein that is quantitatively deficient in the first several months of life is prothrombin. In the coagulation cascade, prothrombin is converted into thrombin, a major regulator of hemostasis. In healthy newborns mean prothrombin values are less than 70% of adult mean values (1), and investigators have found that neonatal prothrombin level is directly proportional to the amount of thrombin generated (1). In fact, the impaired ability of newborn plasma to generate thrombin in the face of deficient prothrombin has been shown to be similar to adults who are being treated therapeutically with an anticoagulant (2). Therefore, low prothrombin levels in neonates have important implications when considering anticoagulant therapy.