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The main components of protein C anticoagulant system are protein C (PC), protein S (PS) and thrombomodulin (TM); the system plays a protective role in pregnancy, mainly because it prevents the utero-placental circulation from local thrombosis. It is unknown whether the protein C anticoagulant pathway exists in amniotic fluid. The aim of the present study is to find out whether these three components are present in amniotic fluid.The study group consisted of 50 parturients with an uneventful pregnancy and birth and 25 non-pregnant controls. Amniotic fluid and blood were sampled at the end of the 1st stage of labor. PC, PS and TM were measured by immunoenzymatic method.All the samples of amniotic fluid contained measurable amounts of antigens of PC, PS and TM, although their concentrations were significantly lower than in the mother's blood: (i) The concentration of PC in amniotic fluid was 6.24 ± 3.50% and PS 2.40 ± 1.64%, while in the mothers' plasma it was 138.26 ± 12.38% and 93.15 ± 13.24%, respectively (P<0.0001). (ii) TM concentration in amniotic fluid constituted 63.92% of the concentration in the mother's blood (2.71±1.21 ng/mL vs. 4.24 ± 0.88 ng/mL, P<0.001).Protein C, protein S and thrombomodulin are physiological constituents of the amniotic fluid. As their concentrations are low, it is reasonable to assume that they cannot counterbalance the procoagulant activity of amniotic fluid.