Deep vein thrombosis (DVT) is the formation of a blood clot in the deep veins of the body. It usually occurs in the deep veins of the leg, where the blood flows more slowly. Blood clots are masses of sticky blood cells that form when a blood vessel is damaged. The body naturally creates blood clots as part of regular maintenance to seal small breaks in a blood vessel and to stop bleeding. Sometimes, due to underlying conditions or medications, blood can develop an increased clotting ability.

A large clot that stays at the site of formation is called a thrombus and it prevents the flow of blood and oxygen to organs, which can cause damage. If part of the thrombus breaks off and travels to other parts of the body through the bloodstream and lodges in other organs, it is called an embolism. Emboli can cause severe complications, such as breathing difficulties or stroke.

So how does being on "the pill" predispose to a DVT? The birth control pill contains different forms of the female hormones estrogen and progestin. Estrogen increases the risk of DVTs by increasing the production of certain chemicals necessary for the blood to clot. It also increases platelet numbers and the stickiness of platelets, which increases clot formation. This same increase in estrogen also occurs naturally in late pregnancy, resulting in a similar or greater increase in the risk of DVTs. Progestin causes the blood vessels to relax and widen, allowing the blood to pool in the veins, increasing risk of clot formation.

Since the birth control pill increases the clotting ability of blood, it is generally not recommended or requires close monitoring by a doctor if you are overweight, have inherited blood clotting disorders, or have a history of DVTs. The risk of clotting also increases with larger doses of estrogen, so it is recommended that women use as low a dose of estrogen as possible.

Also, talk to your doctor if you are on the pill and need to undergo major surgery.

Some people who develop a DVT may have no symptoms at all; however, if you experience any symptoms, such as pain or tenderness, an area that is warm to the touch, or swelling or redness in one leg that cannot be explained by exercise or injury, you should check with your doctor as soon as possible.