Both clinical and laboratory findings suggest that pregnancy constitutes a hypercoagulable condition; yet none of the observed laboratory changes are specific for thrombosis. An essential step involves thrombin-mediated fibrin generation. In the process, fibrinopeptide A (FPA) is cleaved from fibrinogen. Using a radioimmune assay, FPA was determined prospectively in a longitudinal and cross-sectional fashion. Fibrinopeptide A increased significantly over control by the end of the first trimester, from 1.3 ng/ml to 2.8 ng/ml. It continued to increase until 30 to 32 weeks' gestation and then plateaued at 4.3 to 4.7 ng/ml. In the immediate postpartum period, FPA remains elevated. In conclusion, thrombin generation as reflected in FPA production is increased throughout pregnancy, thus confirming a hypercoagulable milieu.