Most birth control pills contain the hormones estrogen and synthetic progesterone, called progestin, and contribute to an increased risk of blood clots, because they cause the level of clotting factors or clotting proteins in a woman’s body to increase.
The pill, the patch or the ring — when they contain estrogen — all do the same thing: They cause the body to hormonally imitate pregnancy, or to think it’s pregnant. In doing so, they prevent pregnancy.
Women’s bodies have evolved over time to offer them some protection from serious bleeding related to pregnancy. Their bodies actually produce more clotting factor proteins, very early on in the pregnancy, so that their blood will clot more readily and protect them from any bleeding challenge that might occur with a miscarriage or during childbirth.
When birth control methods made with pregnancy hormones like estrogen and progestin cause the body to think that it’s pregnant, these same changes will occur, putting women at an increased risk for blood clots.