Influence of changes in levels of coagulation factors and anticoagulants on acquired activated protein C (APC) resistance were studied in 40 healthy women during normal pregnancy. Factor VIII (FVIII), von Willebrand factor antigen (VWF:Ag), free protein S (FPS) and protein C were determined at 5-13, 14-26 and 27-40 weeks gestation and more than 6 weeks postpartum. APC anticoagulant activity was determined by measuring the activated partial thromboplastin time before and after adding human APC, expressed as the APC-sensitivity ratio (APC-SR). During the second and third gestation trimesters a significant increase (P < 0.05) in FVIII and VWF:Ag levels and a decrease in FPS levels were seen compared with the first trimester. Postpartum FVIII and VWF:Ag levels significantly decreased and FPS levels increased compared with the third trimester. Protein C levels remained unchanged during pregnancy and postpartum. Between increased FVIII and lowered APC-SR a trend of inverse correlation (r = -0.329; P = 0.076) occurred in the second trimester. No correlation was found between APC-SR and FPS or VWF:Ag levels. A remarkable finding is the strong inverse relationship between APC-SR and protein C levels (r