Andra James, MD, Duke University
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. Birth control pills, the leading method of birth control in the United States, increase the chance of developing a blood clot by about three- to four-fold. Most birth control pills contain an estrogen and a progestin (synthetic progesterone). 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. Since the risk of a blood clot is reduced by anticoagulation, women who are taking anticoagulants should be allowed to take birth control pills.
Women with thrombophilia or a history of thrombosis who are not taking anticoagulants have fewer choices, but alternative methods are available. One alternative is progestin-only contraceptives. Progestin-only contraceptives include progestin-only birth control pills such as Micronor®, Nor-Q.D.®, and Ovrette®; the levonorgestrel (Mirena®) intrauterine device (IUD); every 3-month injections of medroxyprogesterone acetate (Depo-Provera®); and the new, 3-year implant (Implanon™). While progestin in the higher doses used to treat abnormal vaginal bleeding has been shown to increase the risk of thrombosis five- to six-fold, progestin in the doses used in contraceptives has NOT been shown to increase the risk of deep vein thrombosis or pulmonary embolism.
Women who take anticoagulants are vulnerable to heavy menstrual bleeding and bleeding into the ovary or into the abdomen at the time of ovulation (mid-cycle release of an egg). Half of women who take anticoagulants experience heavy menstrual bleeding. Heavy menstrual bleeding is not a reason to discontinue anticoagulants, however, since it can be managed.
If a woman is on anticoagulation and experiences heavy menstrual bleeding, the full range of treatments may be tried. This usually consists of an evaluation by a gynecologist to make sure there is no abnormality of the uterus or its lining. If there is an abnormality, surgery may be required. If there is no abnormality, hormonal treatments may be tried. Since birth control pills and the Mirena® IUD reduce heavy periods, one or the other may be prescribed. If a woman plans no more children, she may have the lining of her uterus destroyed by a technique called endometrial ablation or she may even have a hysterectomy. Because anticoagulation should be discontinued at the time of an operation, special planning is required if any surgery is performed.
Ovulation is not normally accompanied by any significant amount of bleeding, but in a woman on anticoagulants, the potential exists for bleeding into the ovary and into the abdomen. Bleeding into the ovary is an infrequently considered complication of anticoagulant therapy. Combined hormonal contraceptives with estrogen and a progestin (pills, patches, and rings) prevent ovulation and effectively prevent bleeding into the ovary and abdomen. This is one reason why women who are taking anticoagulants should be allowed to take birth control pills. Estrogen, besides being used to prevent pregnancy, is used to treat postmenopausal symptoms. Postmenopausal hormone therapy consists of an estrogen or an estrogen and a progestin (synthetic progesterone). Postmenopausal hormone therapy increases the chance of developing a blood clot by two- to four-fold. For the average woman taking postmenopausal hormone therapy, the absolute risk of a blood clot is still small. Only one in 300 women per year who are taking postmenopausal hormone therapy will develop a blood clot, but the risk is much higher for a woman who has had a blood clot or a woman with thrombophilia.
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.