The haemostatic (blood clotting) system involves the interaction of proteins in the blood, the blood vessel wall and the flow of blood to control bleeding and blood clotting. Developmental Haemostasis is a term that represents the maturation of the haemostatic system from birth to adulthood. There are differences in the concentration, structure and activity of many proteins involved in blood clotting. These changes play an important role in physiological development and are important in providing appropriate diagnosis and treatment of bleeding and clotting disorders (e.g. thrombosis). The age-specific differences in the blood clotting system may contribute to the fact that children are less prone to developing thrombosis compared to adults. Differences in structure of haemostatic proteins Studies have shown that there are structural differences in some of the major blood clotting proteins in newborns and children when compared to adults. These structural differences can lead to differences in the activity of haemostatic proteins within the blood clotting system, as well as other physiological systems (e.g. immune system). Some examples of age-specific differences in the structure of blood clotting proteins are: Fibrinogen The fetal form of fibrinogen has increased sialic acid and phosphorus content compared to the adult form, affecting assembly of fibrin from fibrinogen to form blood clots. This means that blood clotting takes longer in newborns. Structural differences in fibrinogen extend into childhood, where fibrinogen from children has a higher molecular weight compared to adults. Protein C is active in anticoagulation (anti-clotting) and breaking down blood clots. A newborn form of protein C has been detected and has a higher proportion of single chain molecules than the adult two-chain molecule. Antithrombin is an anticoagulant protein and is important in preventing blood clotting. In animal studies, Antithrombin in newborns has a decreased sialic acid content compared to adults. There are also differences in the way Antithrombin interacts with Heparin, an important anti-blood clotting drug. There is more Antithrombin bound to Heparin in blood from newborns compared to blood from adults, suggesting different structural forms of the protein. The size of the Antithrombin molecules in neonates and children is larger than in adults. Differences in concentration and function of haemostatic proteins Antithrombin: the levels of antithrombin increase with age. Antithrombin levels in newborns are less than 50% of the levels in adults. By six months of age Antithrombin levels increase to adult levels. Antinthrombin is present in various isoforms. There is