why does birth control cause blood clots

Is it true that birth control pills cause blood clots? Birth control pills are the leading method of birth control (contraception) in the United States. Although they do not cause blood clots, most birth control pills do increase a womanБs chance of developing a blood clot by about three to four times. Most oral contraceptives contain an estrogen and a progestin (synthetic progesterone). Estrogen and progesterone have many effects on a womanБs body. They are the hormones that sustain pregnancy and, when given in the form of birth control pills, imitate pregnancy, thereby preventing pregnancy. These hormones also increase the levels of clotting factors and are assumed to be responsible for womenБs increased risk of blood clots during pregnancy. For the average woman taking birth control pills, the absolute risk of a blood clot is very small: Only 1 in 1,000 women per year who are taking birth control pills will develop such a clot.


For a woman with thrombophilia or a history of thrombosis, however, this risk is significantly higher. The new patches (transdermal contraceptives) may increase this risk even more. The amount of estrogen absorbed from the patches has been reported to be 60 percent higher than the amount delivered by the pills. Little information about the risk of blood clots with birth control rings is available. Like patches and most birth control pills, these devices also contain an estrogen and a progestin; thus they probably carry a risk of thrombosis similar to that of birth control pills or patches. The risk of a blood clot is reduced by anticoagulation, so women who are taking anticoagulants may take birth control pills. Women who are not taking anticoagulants have limited choices for contraception, but alternative methods are available. One option is a progestin-only contraceptive. Progestin-only contraceptives include progestin-only birth control pills such as Micronorб, Nor-Q. D. б, and Ovretteб; the levonorgestrel (Mirenaб) intrauterine device (IUD); and every-three-month injections of medroxyprogesterone acetate (Depo-Proveraб).


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 DVT or PE. Read more here:
In Europe and North America, estrogen/progestogen oral contraception has been associated with an increase in venous thromboembolism, myocardial infarction, and stroke. These hazards are found mainly in smokers and in women over the age of 35. Venous thromboembolism appears to correlate with the estrogen dosage, and the arterial complications with both the estrogen and progestogen components. Blood coagulation and vascular thrombosis are intimately related.


Estrogen/progestogen oral contraception affects blood clotting by increasing plasma fibrinogen and the activity of coagulation factors, especially factors VII and X; antithrombin III, the inhibitor of coagulation, is usually decreased. Platelet activity is also enhanced with acceleration of aggregation. These changes create a state of hypercoagulability that, to a large extent, appears to be counterbalanced by increased fibrinolytic activity. Studies of the oral contraceptives in current use show that the coagulation effects depend on the dosage of estrogen and the type of progestogen used in combination. Current research is aimed at finding the estrogen/progestogen formulations that induce the least changes in the coagulation system and other physiologic processes. In this respect, the new low-dose formulations are a major step forward and should reduce the risk of vascular thrombotic complications.

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