Blood coagulation and fibrinolytic inhibitors and the balance between and within the two systems were investigated in 26 normal pregnant women during pregnancy and the puerperium. The concentration of the coagulation inhibitors antithrombin and protein C remained within normal levels, whereas the mean level of free protein S showed a significant decrease from 0.26 U/mL in early pregnancy to 0.14 U/mL in week 35. At the same time, soluble fibrin levels increased from 9.2 to 13.4 nmol/L and thrombin-antithrombin complexes increased from 3.1 to 7.1 micrograms/L; both are indicators of thrombin activity. A concurrent increase in the levels of the fibrinolytic inhibitors plasminogen activator inhibitor-1 and -2 from 7.4 to 37.8 AU/mL and 31 to 160 micrograms/L, respectively, suggests a decrease in fibrinolytic activity. However, the levels of fibrin D-dimer, ie, fibrin split products, also increased in parallel from 91 to 198 micrograms/L, suggesting that fibrinolysis is present. Thus, a balance normally exists, which is probably why thrombotic events are rare during pregnancy.