Establishing Expectancy Values for Fibrin Monomer in Uncomplicated Pregnancy.

During pregnancy, a physiological increase of molecular activation markers (MAM) of hemostasis such as prothrombin fragments 1 + 2, thrombin-antithrombin complex, and D-dimers (DD) occurs. Therefore, monitoring MAM levels during pregnancy to evaluate the risk of venous thromboembolism (VTE) may be unreliable; nevertheless, DD analysis in pregnancy is widely performed. In contrast to DD, fibrin monomer (FM) levels have been reported to remain stable during pregnancy.  The main aim of this study was to define the expected range for FM levels in pregnant outpatients. In addition, we examined the impact of the individual VTE risk, as calculated by the pregnancy risk score of the Royal College of Obstetricians and Gynaecologists (RCOG), as well as that of antithrombotic treatment on FM levels.  A total of 342 pregnant women seen at our hemostasis unit were included throughout 350 pregnancies in 899 samples.  Low-risk thrombophilia, but not the RCOG score itself, was found to influence all MAM levels, whereas antithrombotic treatment had only an impact on DD. For FM, a reference range could be calculated irrespective of the pregnancy term, in contrast to other MAMs, which fluctuated throughout pregnancy.  Our findings suggest a stronger impact of inherited thrombophilia on hemostasis activity during pregnancy as compared with acquired or other predisposing thrombophilic risk factors. FM levels showed a marginal increase during pregnancy in contrast to other MAM and remain a potential candidate to improve the laboratory assessment of VTE risk during pregnancy. Further prospective studies in pregnant patients with suspicion of VTE are needed.