Men may actually have a higher overall risk of thrombosis than women, but women have risks due to pregnancy, birth control and postmenopausal hormone therapy that men do not. These risks are generally attributed to estrogen, a key ingredient in birth control pills, patches, and rings, and in postmenopausal hormone therapy.
Estrogen does not cause blood clots, but it does increase the risk by several-fold.
Estrogen and progesterone have many effects on a woman’s body. They are the hormones that sustain pregnancy and, when given in the form of birth control pills, imitate, and, therefore, prevent pregnancy. Estrogen also increases the levels of clotting factors and is assumed to be responsible for the increased risk of blood clots during pregnancy.
For the average woman taking birth control pills, the absolute risk of a blood clot is still small. Only one in 3000 women per year who are taking birth control pills will develop a blood clot; but for the woman with thrombophilia or a history of thrombosis, the risk becomes substantial.
The new patches (such as Ortho Evra) may increase the risk slightly more, since the amount of estrogen absorbed is higher than is absorbed with the pill. There is little information about the risk of blood clots with the birth control ring NuvaRing®), but, like patches and most birth control pills, they also contain an estrogen and a progestin, and, therefore, probably carry a risk of thrombosis similar to that of birth control pills or patches.
Postmenopausal hormone therapy with estrogen, or with estrogen and a progestin, increases the risk of breast cancer, stroke, deep vein thrombosis and pulmonary embolism. Postmenopausal symptoms such as hot flashes, sleeplessness, vaginal dryness and bone loss can be managed without estrogen. For women who are not taking anticoagulants, but who have had a blood clot or have thrombophilia, the circumstances that would justify taking postmenopausal hormone therapy are rare or nonexistent.
Source: Women’s Health – Blood Clots