Without exposure to a hormone-based contraceptive, the annual risk of venous thrombosis is estimated at between 5 and 10 cases per 100,000 women aged 15 to 44. The risk increases with age in all women. The risk is around 60 thromboses per 100,000 pregnancies. 

Combined oestrogen and progestin oral contraceptives increase the risk of venous and arterial thromboses. The risk of venous thrombosis is around 20 cases per 100,000 women-years with oestrogen + progestin pills when the progestin is norethisterone or levonorgestrel in combination with less than 50  microgrammes of ethinylestradiol per pill. The risk of venous thrombosis is 30 to 40 cases per 100,000 women years when the combined progestin is gestodene or desogestrel (known as third-generation pills).

Likewise, drospirenone, a progestin present in fourth-generation pills, exposes women to an increased risk of both venous thrombosis and arterial thrombosis (myocardial infarction and stroke).

Compared with second- and third-generation pills, the risk of venous thrombosis is also increased with contraceptive skin patches and vaginal rings. Whether these methods increase the risk of arterial thrombosis is uncertain.

In practice, when combined oestrogen + progestin contraception is desired, it is better to use an ethinylestradiol + levonorgestrel combination, which carries a lesser risk of venous or arterial thrombosis.

©Prescrire 1 November 2013

"Thrombotic risk of contraceptive transdermal patches and the contraceptive vaginal ring" Prescrire Int 2013; 22 (143): 266-269. (Pdf, subscribers only)