Hemostatic effects of two oral contraceptives containing low doses of ethinyl estradiol and either gestodene or norgestimate: an open, randomized, parallel-group study.

Objective: To determine effects on blood clotting of two modern low-dose monophasic oral contraceptives.

Subjects and methods: We measured in vivo markers of intravascular coagulatory and fibrinolytic activity in 40 volunteers randomly assigned to one of two low-dose oral contraceptives (OCs) for 6 months; one contained 35 micrograms of ethinyl estradiol (EE) plus 250 micrograms of norgestimate and the other, 30 micrograms of EE plus 75 micrograms of gestodene.

Results: Both formulations increased coagulatory as well as fibrinolytic activity over baseline: circulating reactive products of thrombin increased by 40%, and plasmin activity by 60%, after 3 months of treatment. Six months of OC use increased hemostatic activity substantially over that with 3 months of use. Differences between both OC formulations were marginal and clinically insignificant.

Conclusion: The data suggest an EE-dose-dependent, balanced activation of in vivo coagulation and fibrinolysis in users of currently available, combined OCs. However, there is considerable consumption of coagulation inhibiting factors, suggesting that women with congenital deficiencies of antithrombin III and protein C should not use combined OCs.