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Does Birth Control Raise Blood Clot Risk After a Marathon?

Research shows that taking birth control pills increases the small chance that a woman will develop a dangerous blood clot. So does flying cross country and competing in a strenuous endurance event such as a marathon. How do those risks add up? Does wearing compression stockings or flight socks during air travel blunt the risk? The short answer to both questions is, “We don’t really know.” But recent studies help to shed light.

Blood clots that block blood flow through the veins in the legs are called deep vein thrombosis (DVT). They can cause pain, swelling, redness and warmth. They are sometimes mistaken for a strained calf muscle. But people often feel no symptoms.

In a few cases these clots break away from the wall of the vein and travel through the blood stream to an artery in the lung. This blockage, called a pulmonary embolism, may cause sudden shortness of breath, chest pain or a cough, and can be fatal.

Air travel, exercise and birth control

Blood clots can form in any athlete for a variety of reasons. Two of the leading causes are air travel and strenuous endurance exercise, such as a marathon. In female athletes, birth control pills also increase risk.

  • Air travel. Flying increases clot risk by two- to four-fold, likely because of inactivity during flight combined with dehydration from dry cabin air and cramped seating that together slow blood flow in the legs. That’s why people on long flights are advised to move around, walk the aisles and drink plenty of water.
  • Sustained exercise. In most people, exercise helps to offset clotting risks because it activates substances that are natural blood thinners. But among men and women who do strenuous endurance exercise, the risk of developing a clot goes up, not down. Doctors have reported numerous cases of clots developing in healthy endurance athletes, including in a half-Ironman triathlete and a cross-country runner.
  • Air travel plus sustained exercise. A 2012 study showed that flying more than four hours after a marathon increased the risk of blood clots in male and female athletes.
  • Birth control pills. According to a large, careful 2013 analysis, birth control pills increase the risk of blood clots by up to four-fold. Out of 1,000 women who take birth control pills for a year, about one will develop a blood clot and 999 will not. This risk goes up for people who have medical conditions or genetic risk factors (such as factor V Leiden) that contribute to clot formation and in people who have had a recent injury or surgery.

Compression socks help—usually

How great is the risk of a blood clot when an athlete’s lifestyle combines all three risks—long-distance travel, intense endurance competition and birth control pills? Can compression socks cut that risk? Those were the questions doctors wanted to answer when they studied women competing in the 2015 Boston Marathon.

There is good research showing that elastic compression stockings can reduce the development of DVT when they are worn during airline flights lasting more than five hours. Compression socks apply gentle pressure to the leg, with more at the ankle and less as they rise to the knee. This improves blood flow from veins near the surface of the skin to deeper veins which return the blood to the heart.

Before and after the 2015 Boston Marathon, researchers looked at blood samples of 29 female athletes for signs that indicate blood clotting. All of the women were using birth control pills, flew more than four hours cross country and ran in the marathon. About half of the women wore compression stockings during the flight to Boston, and the other half did not.

The researchers compared blood samples from these women taking birth control pills who traveled by air with samples from a similar group of women who drove less than two hours to Boston to run the marathon and did not use birth control pills or wear compression socks.

Surprisingly, taking birth control pills did not add to the risk of blood clots; signs of clotting activity increased similarly in those who took birth control pills and in those who did not. Also, contrary to expectations, wearing compression socks seemed to increase the risk of blood clots, rather than reduce it.

Bottom line:

  • Blood clot risks. Extended air travel and sustained exercise such as running a marathon increase the risk of a blood clot. It is reassuring that birth control pills may not increase the risk even more. However, additional studies looking at women with all three risk factors will be needed before researchers can say with confidence how much each risk factor adds to a woman’s overall risk of having a blood clot.
  • Blood clot prevention. Compression socks have been shown to significantly reduce DVT in most airline passengers. Additional research will need to explore their protective effect among endurance athletes, including in women who use birth control pills and those who do not. The best time to wear compression socks—whether in flight before or after an event or during the marathon itself—also needs more study.