Abstract
Oral contraception with conventional oestrogen/progestogen preparations produces raised levels of clotting factors and increased platelet aggregation. Although these changes do not involve as broad a spectrum of clotting factors as thrombosis and the third trimester of pregnancy, they do not appear a desirable side effect and may be responsible for the increased risk of thrombosis. Oral contraception with progestogen alone does not appear to cause similar clotting and platelet changes.