There are two types of systems that inhibit blood clotting. Abnormalities in these systems can lead to thrombophilia, an increased risk of thrombosis.
The first type of abnormality, which concerns a deficiency in substances that stop blood clotting at the right time, is referred to as Activated Protein C resistance (APC resistance). Protein C, protein S (a cofactor for protein C) and antithrombin are involved in stopping the blood clotting process. Consequently, if there is a defect in one of these proteins, there is an increased risk of the blood continuing to clot and therefore an increased risk of thrombosis. A relatively common abnormality in clotting factor V, known as Factor V Leiden, also increases the risk of clotting.
The second type of abnormality concerns situations in which the level of substances that stimulate blood clotting is too high. Several clotting factors are involved in the formation of a blood clot. However, a relatively common abnormality in factor II (prothrombin) has been identified that leads to an increased prothrombin level. Prothrombin is transformed into the active enzyme thrombin (factor IIa), which is one of the most important clotting factors. If the prothrombin level is raised, the clotting system is more active and more difficult to stop, resulting in an increased risk of thrombosis.
Your individual risk of thrombosis if you take contraceptives that contain oestrogen may be partly explained by genetic variations. It is known, for example, that variations in the F5, F2, SERPINC1, PROC and PROS1 genes are partly responsible.
These genes are involved in blood clotting. If a specific variation is present, this influences the blood clotting process, resulting in an increased risk of thrombosis.