Summary

Protein S, protein C and antithrombin are important regulators of coagulation. While deficiencies of these proteins have been linked to adverse pregnancy outcomes, testing for these deficiencies during pregnancy is limited by the use of non-pregnant reference ranges and a limited understanding of the changes that occur during pregnancy. Although several small studies have previously reported on the activity of these proteins during pregnancy, potentially important changes have been overlooked by continuing to compare the activity during pregnancy with non-pregnant reference ranges. In the current study, we investigated the activity of protein S, protein C and antithrombin during the first half of pregnancy in 440 otherwise asymptomatic women who went on to have uncomplicated singleton pregnancies. Consistent with previous studies, we found that antithrombin activity remained unchanged, while protein S activity decreased significantly to a mean level of 46%. We did not observe a progressive decrease in protein S during the second trimester as several studies have suggested previously. In contrast, we observed a potentially biologically significant increase in protein C activity throughout the first 22 weeks of pregnancy. Given the physiological role of protein C, we postulate that this increase may play a role in maintaining early pregnancy through both an anticoagulant and an inflammatory regulation pathway.