Fibrinopeptide A (FPA) is the first peptide released from fibrinogen upon thrombin action. Plasma FPA is cleared rapidly with a first order kinetics and therefore its level reflects the rate of thrombin cleavage of fibrinogen. A prospective study was undertaken to establish normal values of FPA during pregnancy. The mean FPA for the pregnant group (n = 136) was 2.8 ng/ml (SD = 3.3) while it was 1.24 ng/ml (SD = 0.4) for a nonpregnant control group of healthy women (n = 30). The median FPA for the pregnant group was 2.2 ng/ml and 1.4 ng/ml for the nonpregnant group (Wilcoxon test P less than 0.0001). Plasma FPA levels increased with gestational age. The median value was 1.5 ng/ml in the first trimester (n = 18), 1.8 ng/ml in the second trimester (n = 40), and 2.5 ng/ml in the third trimester (n = 78). Plasma FPA concentrations in the third trimester were significantly higher than in the first and second trimester. These findings suggest increased thrombin activity and fibrin generation during the course of normal pregnancy.