Coagulation effects of oral contraception.

In Europe and North America, estrogen/progestogen oral contraception has been associated with an increase in venous thromboembolism, myocardial infarction, and stroke. These hazards are found mainly in smokers and in women over the age of 35. Venous thromboembolism appears to correlate with the estrogen dosage, and the arterial complications with both the estrogen and progestogen components. Blood coagulation and vascular thrombosis are intimately related. Estrogen/progestogen oral contraception affects blood clotting by increasing plasma fibrinogen and the activity of coagulation factors, especially factors VII and X; antithrombin III, the inhibitor of coagulation, is usually decreased. Platelet activity is also enhanced with acceleration of aggregation. These changes create a state of hypercoagulability that, to a large extent, appears to be counterbalanced by increased fibrinolytic activity. Studies of the oral contraceptives in current use show that the coagulation effects depend on the dosage of estrogen and the type of progestogen used in combination. Current research is aimed at finding the estrogen/progestogen formulations that induce the least changes in the coagulation system and other physiologic processes. In this respect, the new low-dose formulations are a major step forward and should reduce the risk of vascular thrombotic complications.