Do the Side Effects of Birth Control Include Blood Clots? Here's What Science Says

You might have seen the warning in small print: Your birth control pill prevents pregnancy, but it also increases your chances of developing a blood clot.

The chances of it happening aren't all that high. For average, healthy people taking birth control pills, the risk of developing a blood clot is roughly five in 10,000, according to various estimates. Chances of developing a clot are higher in some newer birth control pills like Yaz — around 14 in 10,000 — but as NBC News points out, "the overall risk is still very low."

But if they do develop, the results can be fatal. Just last year, a 21-year-old British woman died of blood clotting linked to the birth control pills she'd started taking just a few weeks prior, according to the Independent. Another woman wrote in Shape that her birth control pills — the likely cause of a blood clot in her lung — "almost killed" her.

Why do birth control pills increase your risk of developing potentially fatal blood clots — and is it really something people need to worry about? In honor of Deep Vein Thrombosis Awareness Month — yep, it's a real thing — Mic enlisted Dr. Thomas Maldonado, vascular surgeon and associate professor in the Department of Surgery at NYU Langone Medical Center, to explain the facts.

What are blood clots, anyway? Blood clots are masses of coagulated blood formed by platelets and certain proteins in the bloodstream. Sometimes blood clots are useful — lifesaving, even — as they help stop excessive bleeding when we're injured. 

But they can also be dangerous. A condition called deep vein thrombosis occurs when a blood clot forms in a major vein — such as those in your legs and pelvis — and cuts off blood flow. If one of those blood clots becomes detached from its spot in a vein, it can travel up toward the lungs "like a pebble in a stream," Maldonado told Mic. If a clot becomes lodged in one of the arteries there — a condition called pulmonary embolism — blood may not be able to reach the lungs.

"This is one of the causes of sudden death," Maldonado said.

How do you know if you're experiencing a blood clot? People with deep vein thrombosis will often has pain or swelling of the leg. Leg pain doesn't always mean you have a blood clot, but "when you're on birth control, it's something you have to take seriously," Maldonado said.

If a blood clot has traveled to the lungs, patients may notice chest pain or a shortness of breath.

If medical tests — such as an ultrasound or CT scan — reveal that a patient has a blood clot, blood thinners will often solve the problem. "They prevent the clot from propagating," Maldonado told Mic. "The clot will break up and the vein will open up again."

So why are birth control pills linked to blood clots? Birth control pills don't directly cause blood clots, but they do increase your likelihood of developing them. Research has shown that people taking older forms of the pill — those than contain levonorgestrel and norgestimate — are around 2.5 times more likely to get blood clots than people who are not on birth control at all, according to CBS News. On newer forms of the pill you're around 4.3 times more likely to get them than someone not on birth control, according to BuzzFeed.

Most birth control pills contain estrogen and progestin. Estrogen increases the levels of clotting elements in the blood, according to the National Blood Clot Alliance. That's why people are also more likely to develop blood clots during pregnancy. In fact, your risk of developing a blood clot while pregnant or during the postpartum period is actually higher than it is when you're on birth control pills, BuzzFeed points out. 

Should we consider the risk of blood clots when choosing a contraceptive method? "I don't think this is a risk that's severe enough to warrant people abandoning oral contraceptives," Maldonado said.

That being said, before getting on the pill, it's important for people to know if they're at higher risk than others for developing blood clots. "I do think people need to be aware of what their risk profile is," Maldonado said.

"If you're a smoker who has a strong family history of deep vein thrombosis, whose mother or father had a pulmonary embolism, who maybe is sort of sedentary sometimes," he continued, "these are all things that have to be considered when you're starting a birth control pill."

Of course, you could just opt for an intrauterine device — an IUD — instead.