Risk of venous thromboembolism and stroke associated with oral contraceptives. Role of congenital thrombophilias.

To assess the risk of thromboembolism in women using oral contraceptives (OCs), we identified through computer search in the hospitals of the province of Parma, Italy, all women aged 15-44 who were resident in the province and had a documented thromboembolic event in the years 1989-93. The number of users and nonusers of OCs was estimated by the drug sale data for the province and by the demographic statistics. In cases with venous thromboembolism (VT) the prevalence of concomitant deficiency of antithrombin III, protein C, protein S, and of factor V gene mutation Arg506GIn was evaluated. The incidence rate of VT was 37/59,603 woman-years in users (0.62 per 1000) and 13/303,954 woman-years in nonusers (0.042 per 1000), for a relative risk (RR) of 14.5 (95% confidence interval: 7.8-27.1; P < 0.001); the rate of stroke per 1000 woman-years was 0.17 in users and 0.036 in nonusers (RR = 4.6; 2.9-10.7; P < 0.01). A congenital thrombophilia involving the protein C anticoagulant system was documented in about 25% of young women developing venous thromboembolism while on OCs.