Background: We wanted to study the incidence of venous thromboembolism (VTE), acquired risk factors of VTE and preventable cases among users of combined oral contraceptives (COCs).
Methods: All women aged 15-44 years, (n = 24 373) living in the county of Jämtland, Sweden, between 1991 and 2000, constituted the study base in a retrospective case-reference study. Women with VTE were identified through hospital registers and interviewed by telephone. The utilization of COCs according to age was obtained from a prospective prescription database, and data from national health databases were used.
Results: Of 88 women with first-time VTE, 43 (49%) were COC users and 13 (15%) were pregnant. All women had at least one known risk factor, and 51 (58%) women had combinations of risk factors. The total incidence rate of VTE per 100,000 women-years for all women were 36 (29-44), for nonusers 19 (12-25) for women using third generation COCs 115 (67-184), for women using other COCs 60 (37-83), and for women during pregnancy and postpartum 103 (55-177). Of the total 244,000 women-years represented, COC users constituted 24%, pregnant women 5%, and women with other acquired risk factors 5%. The corresponding incidence rates after excluding VTE cases with other acquired risk factors were 10 (6-14), 1.2 (0.14-4.4), 64 (29-121), 27 (13-48), and 59 (24-121), per 100,000 women-years. In 11 (26%) of the COC-related VTE cases, there were relative contraindications for use of COCs or lack of thromboprophylaxis in relation to surgery. CONCLUSION. We found a very low incidence of idiopathic VTE among young non-OC users. The incidence of VTE during pregnancy was only slightly higher than during COC use. It was considered that a significant part of COC-related VTE might have been avoided.