- Key words: fetus, newborn, hemostasis, coagulation.
- Page(s) : 491-9
- Published in: 1998
Normal hemostasis is a dynamic system that gradually evolves throughout intrauterine life in the fetus, the neonate and then during childhood toward the adult state, and it always remains well balanced at every age of life.
Fetal hemostasis was mainly studied from mid-pregnancy to birth on blood obtained by direct puncture of the umbilical vein under high-resolution real-time ultrasound. Primary hemostasis is efficient from the first trimester of intrauterine life to birth. Fetal platelet counts are similar to that observed in adult with a complete expression of major platelet glycoproteins and antigens, explaining the possibility of early thrombocytopenia in case of alloimmunization. Plasma concentrations of von Willebrand factor and high molecular weight multimers also contribute to an efficient primary hemostasis explaining the short bleeding time in newborn. Prothrombin time, activated partial prothrombin time and thrombin clotting time, are prolonged in fetuses throughout intrauterine life, and this is explained by low levels of vitamin K-dependent factors (II, VII, IX, and X), contact factors (XI, XII, prekallikrein, and high molecular-weight kininogen), factors V, VIII and fibrinogen. Low levels of antithrombin III, protein C and protein S, heparin cofactor II, and tissue factor pathway inhibitor are also found, probably contributing to a satisfactory hemostatic balance. An increase in factor levels is observed after the thirty-fourth week of intrauterine life for most of the coagulation activators and inhibitors, but only factors V and VIII reached adult values at birth. The fibrinolytic activity is increased in the fetus and variable in neonate despite low plasminogen levels.
During the last month of intrauterine life and in the first hours following delivery, important modifications of the coagulation system occur due to a significant hepatic maturation.
In healthy fetuses or neonates, this particular hemostatic equilibrium provides a good protection from hemorrhage and thrombosis. However, the immaturity of the hemostatic system in the very youngs particularly if premature, does leave them vulnerable to some disorders, such as hemorrhagic disease of the newborn due to vitamin K deficiency or disseminated intravascular coagulation