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Illustration by Marta Pucci

Sex

Hormonal birth control and blood clots

by Maegan Boutot, Science Writer for Clue; and Nicole Telfer, Science Content Producer
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Top things to know:

  • Blood clots aid in the healing process. Proteins in your bloodstream activate to form a makeshift plug—called a blood clot or thrombus—over an injury or hole.
  • Blood clots in veins and arteries can be dangerous, because they can partially or completely block the flow of blood.
  • Sometimes, clotting occurs in areas it’s not supposed to, and can cause damage. This is what happens in the case of Deep venous thrombosis (DVT).
  • People who use hormonal birth control methods have a higher risk of blood clots compared to people who don’t use them.

Hormonal birth control and blood clots

Hormonal birth control, like the pill, increases your risk of blood clots. If you’re interested in using hormonal birth control, it’s important to understand your personal risk. Your risk changes with age and with the development of certain medical conditions, but most premenopausal people have a low underlying risk of developing clots.

What your healthcare provider might say

People who use hormonal birth control methods that contain estrogen have higher risk of blood clots in comparison to people who do not use hormonal methods, but the overall risk is still very low among most people using hormonal methods. Types of birth control that contain estrogen are called combined hormonal contraceptives and include the combined-hormonal pill, the ring, and the patch.

Birth control methods that only contain a progestin (synthetic progesterone) probably do not increase a person’s risk of blood clots, but this may depend on the method.

Blood clots are very rare among people who are not pregnant and are under 45 years old—even among those who use estrogen-containing birth control. The risk of blood clots is comparatively very high among pregnant people and people who have recently given birth. It’s important to balance the risk of blood clots and other negative side effects with the benefits of estrogen-containing birth control.

Here’s why it’s tricky

The amount of estrogen and the type of progestin in a form of birth control probably affects the risk of blood clots. There have been many formulations of the combined-hormonal pill since it was developed in the 1970s, and so studies done at different time points and among different populations have found different results. They generally all find an increase in risk, but the size of the risk differs among studies and among the different types of pills.

The patch, ring, and progestin-only forms of contraception, like the contraceptive shot and hormonal intrauterine device (IUD), have not been studied as much as the combined-hormone pill. It is thought the patch and ring increase the risk of clots, but more research is needed.

Most forms of progestin-only contraception are not associated with increased risk of blood clots. The contraceptive injectable or “the shot” may be associated with increased risk of blood clots, but more studies are needed.

Here’s what the research says about each birth control type

What are blood clots, anyway?

Blood is transported throughout the body through a series of tubes known as blood vessels. When blood is moving away from the heart, these vessels are called arteries, which bring oxygen-rich blood from the lungs (via the heart) to the entire body. Once this blood has reached its destination, gas exchange occurs across blood vessels (known as capillaries) with the surrounding tissue, and the now de-oxygenated blood returns to the heart through blood vessels known as veins.

In a healthy body, blood stays within this vast network of blood vessels, and is not free flowing under your skin. It's a very controlled system. If any injury or breakage occurs to a blood vessel, there is a safety system in place to patch any holes up quickly and effectively—this is called clotting. Think about when you get a cut: you don’t bleed indefinitely. Within minutes blood flow will slow and stop from your cut—this is thanks to clotting. Proteins in your bloodstream activate to form a makeshift plug—called a blood clot or thrombus—over the injury or hole. Blood clots can occur in both arteries and veins.

Sometimes, clotting occurs in areas it’s not supposed to, and can cause damage. Types of dangerous blood clots include:

  • Deep venous thrombosis (DVT): clots that form in the muscles, often in the leg
  • Pulmonary embolism: blockages in the arteries of the lungs caused by clots, often ones that started as a DVT

DVT and pulmonary embolism both fall into a category of conditions that are known as venous thombroembolism (VTE) (1).

Blood clots that travel to the brain or the heart can cause strokes or heart attacks, respectively.

(If you want more information on the types of blood clots and how frequently they happen, see the bottom of the article.)

Combined oral contraceptives (the pill)

Combined oral contraceptives (the pill) contain a form of estrogen and a synthetic progesterone (called a progestin). Premenopausal people using the pill are more likely to experience a VTE than premenopausal people not using hormonal birth control (2,3), but the overall risk is still small.

The estimates of the risk of VTE vary, but researchers think that premenopausal people using the pill age 15-44 are 1.5 to 7 times more likely to experience a VTE than premenopausal people not using hormonal contraception (2-6). People using the pill are also about 1.3 to 2.5 times more likely to experience a stroke or heart attack than those not using the pill (7,8). Some combinations of estrogen and progestin may carry a higher risk for stroke or heart attack (7,8), but more studies are needed.

Although this is a large relative increase, the number of people affected is still small. For example, in a large study using Danish health registry systems, the rate of VTE was about 4 in every 10,000 premenopausal people age 15-44 not using any form of hormonal contraception per year. In comparison, the rate among pill users of the same age group was about 4 to 16 per 10,000 per year (5), depending on the type of pill.

The amount and type of hormones in pills have higher risk of blood clots than others. As the amount of estrogen in a pill increases, so does the risk of all types of blood clot (3-7).

The risk of blood clot may also depend on the the type of progestin. Pills that contain the progestin levonorgestrel tend to have lower risk of VTE as compared to pills that have the progestins desogestrel, gestodene, drospirenone, or cyproterone acetate (3,6).

The patch, the ring, the shot, the IUD, and other forms of contraception

The contraceptive patch and vaginal ring are combined hormonal contraceptives, meaning they have both an estrogen and a progestin. Similar to combined hormonal pills, these forms of birth control may increase the risk of VTE and potentially stroke (7,9). The risks associated with this forms of birth control also may be higher than the risks for pills. A recent literature review found that those using the patch or ring may be at higher risk of VTE as compared to those using combined hormone pills, but there needs to be more research to understand if this is true (9).

Progestin-only contraception are birth control options that contain only a progestin, so it doesn’t contain estrogen. Progestin-only methods include the contraceptive shot (such as DMPA or Depo), the hormonal intrauterine device (IUD), the contraceptive implant, and progestin-only pills.

Less is know about the risk of VTE among users of progestin-only methods. It is currently thought that progestin-only pills, the implant, and the hormonal IUD generally do not increase a person’s risk of any type of blood clot or stroke (5,7,8,10).

In comparison to the other forms of progestin-only contraception, the contraceptive shot may increase risk of blood clots, particularly VTE; however, there have only been a few studies and not all of the findings agree on this association (10,11). More research is needed to see if these results are true or if they are due to another factor. For example, it’s possible that people who are advised not to use combined-hormonal methods due to their increased risk of blood clots may chose to use the shot (10).

Balancing the risks and benefits of hormonal birth control

Hormonal birth control has many benefits. For people having penis-in-vagina sex, one of the most important benefits is that it greatly reduces the risk of unintended pregnancy as compared to not using any form of contraception.

Pregnancy and the postpartum period (ie. the 12 weeks after delivering a baby) come with many risks, including increased risk for blood clots (12). For example, the risk of VTE among postpartum people is about 40 to 65 out of every 10,000 postpartum people per year (13). This risk is more than two times greater than the risk of VTE among pill users.

Some people are at increased risk of blood clots when using combined hormonal contraceptives. People who have multiple risk factors for clots, strokes and/or heart attack should avoid using combined hormonal contraceptives. These risk factors include having recently given birth, superficial venous thrombosis, hypertension, vascular diseases, migraines with aura, and previous DVT (14,15). People older than 35 and who smoke 15 or more cigarettes per day are also advised not to use combined hormonal contraceptives (14,15). Your healthcare provider will be able to tell you if combined hormonal contraceptives are safe for you to use.

It’s important for you to understand the risks and benefits of using hormonal birth control. You can use Clue to track changes to your body when using hormonal birth control to see if the benefits of using your method outweigh any concerns you have. More information on blood clots A blood clot can be superficial, which means the clot is in veins or arteries close to the skin surface, or deep, where the clot forms in veins and arteries in the muscles (1).

Blood clots in veins and arteries can be dangerous. They can partially or completely block the flow of blood. This is called ischemia. If a clot blocks the flow of blood to a major organ, like the lungs or heart, this can cause death. Clots and death due to clots are very rare among people younger than 45 years old.

Deep vein clots and clots in the lung

When clots form in the deep veins, it is called deep venous thrombosis (DVT). DVT often form in the legs, but they can form in other parts of the body too (1). Not everyone with a DVT will show symptoms, but the classic presentation is sudden pain in one leg or calf, pain on walking or standing, swelling, redness, and warmth in the area (1).

DVT can cause serious injury to the lungs if a piece of a clot breaks off from the DVT and becomes free to flow in the bloodstream. This is called an thromboembolus. If an thromboembolus continues to flow downstream of a vein, it’s path will be eventually stopped by the small blood vessels of the lungs and can block blood flow to that area. This is called a pulmonary (lung) embolism (blockage) and can cause symptoms of chest pain and trouble breathing, and can be life-threatening (1). DVT and pulmonary embolism both fall into a category of conditions that are known as venous (vein) thombroembolism (clot-blockage) (VTE).

VTE and serious complications from are very rare among premenopausal people age 15-44 (2). It is estimated that about 4 to 10 in 10,000 premenopausal people age 15-44 who are not using hormonal birth control experience a VTE in one year (16,17). The risk of VTE increases with age (5,16).

Strokes and heart attack from blood clots

Blood clots in the brain can cause a stroke. Blood clots in the heart can cause myocardial (heart) ischemia (decreased blood flow), which can cause a heart attack, also known as myocardial infarction. Blood clots in the brain and the heart are more rare than VTE events among premenopausal people. Less than 5 in 10,000 premenopausal people age 15-44 experience a stroke or a heart attack each year (7.16).

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