Birth control pills are the leading contraceptive method in the United States, where they are used by an estimated 14 percent of women between the ages of 15 and 49. Though birth control pills have been widely used for decades and are proven safe and effective, 1 in 3,000 women per year who take them will develop a blood clot.

Birth control pills do not cause blood clots directly, but the National Blood Clot Alliance states that this contraceptive method increases a woman's risk for developing a blood clot by a factor of three to four. Still, the absolute risk is low in carefully selected patients, and the risk of a blood clot during pregnancy is approximately double that of getting one while taking birth control.

Blood clots can form in the veins of the legs, arms and groin and move to other parts of the body. If a blood clot reaches the lungs,