To research the association of prothrombin (factor II) activity given the prothrombin G20210A mutation carriage with its clinical manifestations as thrombotic complications.A prospective clinical cohort study of 290 women of reproductive age was conducted. Two cohort groups were identified: the study group of 140 patients with prothrombin mutation G20210A genotype and the control group of 150 women with G20210G genotype.The prothrombin G20210A mutation carriage is associated with the risk of thrombotic complications compared to the wild G20210G type (RR =17.1; <0.0001) and is characterized by thrombosis localized both in the venous (66.7%) and arterial (33.3%) vascular pools. The threshold value of prothrombin activity (174.8%) for G20210A genotype was calculated, making it possible to conclusively predict the risk of thrombotic events with the accuracy of 90.4%.The phenotypic manifestation of the prothrombin G20210A mutation in the form of venous and arterial thromboses in women of reproductive age is associated with a super-threshold increase in prothrombin (factor II) activity, which makes it possible to stratify the patients into the group of high risk of thromboses.