5,10-methylenetetrahydrofolate reductase (MTHFR) is the key enzyme in folate, methionine and homocysteine metabolism. The disturbances in MTHFR activity could be the cause of increased serum level of homocysteine. Hyperhomocysteinemia is a risk factor of changes in coagulation cascade through direct cytotoxic influence on endothelium, atherogenesis, activation of coagulation factor V and VII, increased level of thrombin and platelets aggregation. Genetic disturbances in MTHFR enzyme activity in the presence of polymorphic variants of its gene are responsible for homocysteine augmentation and could be the reason of several gestational complications such as recurrent miscarriages.