HEMOSTATIC CHANGES AND THE ORAL-CONTRACEPTIVE PILL

Oral contraceptives have been linked to an increased incidence of thro mbovascular disease. This may be mediated by their effects on the haem ostatic system. An increase in the activity of coagulation Factors VII , X and fibrinogen occur with pill usage. Increased Factor VII levels are dependent on both the oestrogen and progestogen component of the o ral contraceptive. A reduction in antithrombin III levels has also bee n observed in some but not all studies. Increased fibrinolysis has als o been shown in oral contraceptive users which should balance the chan ges in the coagulation pathway. The increase in fibrinolytic potential is thought to be due to a decrease in the levels of plasminogen activ ator inhibitor I combined with an increase in the levels of plasminoge n; tissue plasminogen activator antigen is decreased in most studies. The increased levels of endpoints of coagulation and fibrinolysis in p ill users indicate that enhanced activity of both systems is occurring in vivo. The increased coagulation activity appears to be balanced by the rise in fibrinolytic activity, so preserving haemostatic balance. Enhanced platelet activity has also been shown in women taking oral c ontraceptives. Thrombus formation can result, however, when local vasc ular wall damage exists, or when other risk factors for thrombo-emboli sm, such as older age and smoking, coexist and create a local activati on resulting in a thrombus. In these situations, the small differences in levels of coagulation factors in women taking different oral contr aceptive formulations may be important. Pills containing the lowest do ses of oestrogen (20 mu g ethinyloestradiol) have shown the least chan ges in haemostatic factors. The progestogen component of the pill modi fies the effect of oestrogen on the haemostatic system.