Blood coagulation proteins were determined in 285 healthy fetuses from 19 to 38 weeks' gestation and compared with those of 60 normal full- term newborns and 40 adult controls. Prolongation of the coagulation screening tests, prothrombin time, activated partial prothrombin time, and thrombin clotting time, in fetuses throughout intrauterine life was explained by low levels of vitamin K-dependent factors (II, VII, IX, and X), contact factors (XI, XII, prekallikrein, and high-molecular- weight kininogen), factor V, factor VIII, and fibrinogen. Low levels of antithrombin III, heparin cofactor II, protein C and protein S, and tissue factor pathway inhibitor were also found, and these probably contributed to a satisfactory hemostatic balance. Some of these parameters were evaluated by both immunologic and functional assays to detect possible “fetal” proteins. An increase in factor levels was 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. This study therefore showed that fetal hemostasis is a dynamic system that evolves gradually toward the neonatal state and then toward the adult state.