Birth control causes blood clots. Right?
Not as much as having a baby . . .
A few years ago, I had a patient scheduled for repeat cesarean delivery. As the doctor on call, I took a few minutes to review her medical records before meeting her. Brianna’s* pregnancy had been uncomplicated. No high blood pressure or diabetes. Her first cesarean birth, three years prior, had also been uncomplicated with no excess bleeding or infection. In fact, there was only one significant thing in her medical history. When she was 19 years old, she had been diagnosed with a blood clot in her lungs . . . while she was taking birth control pills.
*not her real name
The electronic medical record told me that Brianna had been treated for her blood clot with many weeks of blood-thinning medication. She had tested negative for genetic (inherited) conditions that increase the risk of blood clots, and she had not had any other related problems afterward. Now in her mid-30s, her obstetrician had prescribed a once-daily blood thinner called enoxaparin for most of her pregnancy, then switched to a twice-daily blood thinner called heparin at 37 weeks of pregnancy in case she went into labor ahead of her scheduled cesarean. Seemed simple enough. I closed her chart and made my way to the pre-operative area.
I introduced myself to Briana and her partner, Dave** as the doc of the day who would be performing the cesarean. After the usual pleasantries, I asked if it was ok to confirm some items in her medical history.
“Any complications during this pregnancy?”
None.
“Any contractions, leaking fluid, or bleeding in the past 24 hours?”
No. Nothing.
They were expecting a boy. They wanted circumcision. Brianna hoped to breastfeed exclusively for the first few weeks and then supplement with formula.
“And when was the last time you took your heparin?”
“I didn’t.”
I assumed she had misunderstood me. “Well I know you didn’t take it today”, I chuckled awkwardly, “but when was the last time you took it?”
Brianna and Dave exchanged a look before she repeated herself. “I didn’t.”
“At all?” I could feel that my face was not doing a good job hiding my surprise.
“That’s right. I said I would, but then I didn’t. Is that a problem? Does it affect things today?” She was looking at her hands now, nervously twiddling her fingers.
“No . . .”, I spoke slowly and kept my tone even, trying to remember if I had just missed a note in the chart about Brianna declining the medication. “It does not affect things for the cesarean. But the risk of getting a blood clot is increased during pregnancy, especially in the six weeks after you give birth, more so if you have had blood clots in the past, so I do think it would be helpful to have a conversation with your OB about your plans for the next few weeks, especially with your history.”
“No I know, and I’ll definitely take it after. I just didn’t use it so far.”
I decided not to remind her that it seemed she had said the same thing about using the blood thinner during pregnancy. It was ultimately her choice, after all. And I had told the truth - her decision not to use the medication did not change anything about how I would perform her surgery.
**not his real name, either
The birth went well. Brianna and I did not meet again and aside from sending a note to her primary OB suggesting an open discussion about Brianna’s goals and her decision-making around using the medication, I was not involved her in care, but I thought about her for weeks after. Why verbally agree to take the enoxaparin, but not pick up the prescription? I could not help imagining a scenario where she came in to the hospital with chest pain or trouble breathing. I would consider a blood clot for any pregnant person with those symptoms, but would likely move it to the bottom of my list of possibilities (or differential diagnosis) because if their medical record said they were already on a preventive medication. I was curious about her decision not to use blood thinners. Was she afraid of needles? Was daily medication too much of a burden? Did she just like it wasn’t necessary? She had been on birth control when she was diagnosed with her clot at 19. What if she believed, as many people do, that the birth control was the biggest risk factor in her life for a blood clot and that if she was no longer using hormones, there was nothing to worry about.
I know what you are probably thinking. “Birth control is notorious for leading to blood clots. Of course, it was the reason for Brianna’s clot and it makes sense for her to feel safe from clots as long as she doesn’t take those pills”
Here’s the thing . . . while that that idea is very common, it is not supported by research.
According to the CDC, venous thromboembolism (or VTE: blood clots most often in the legs or lungs) affects 900,000 Americans each year. That’s about 1 in every 370 Americans. 1
Most blood clots are thought to be due to one of three reasons:
Slow blood flow
Injury to the walls of veins (from smoking, for example)
Changes in the blood that make it more likely for form clots. These can be genetic (inherited) or acquired changes. Acquired changes can be due to immobility, surgery, cancer, estrogen (in birth control or other hormone therapies) and pregnancy.
YES. Being pregnant is a risk factor for VTE.
VTE occurs in
1 out of every 1000 pregnancies2 3
5–20 per 10,000 (0.5-2 per 1000) person-years*** in pregnancy.
40-65 per 10,000 (4-6.5 per 1000) person-years during the postpartum period. This risk is increased for up to 12 weeks after giving birth.
3-15 per 10000 (0.3-1.5 per 1000) person-years of combined hormonal contraceptive use
and
1-5 per 10000 (0.1-0.5 per1000) person-years among people who do not use contraception.4
So not only is being pregnant a risk factor for VTE, but you are actually at higher risk of getting a blood clot in your legs or lungs when you are pregnant and in the first 2-3 months after giving birth than when you are on hormonal birth control.
It’s interesting then that you will hear people give advice to avoid birth control or hormones because of the risk of blood clots (as in the text my sister forwarded to me one day near the end of the residency, though that is a story for another time), but I’ve never heard anyone suggest that women should not have babies because of the small, but real risk of a blood clot. Even though the risk of a bloot clot is higher with pregnancy than when one uses hormones (and really, when using methods that contain. estrogen. Most progesterone-only contraception is linked to lower risk of clots).5
I’m not saying anyone needs to take birth control if they don’t want to. I’m not even saying concern about blood clots is a bad reason to choose not to take hormones. We all assess risk differently, and depending on your individual goals and values, your other options, and how concerning the idea of blood clots is to you, you may choose to use some other method to prevent pregnancy (Or treat your symptoms of bleeding/acne/headaches or whatnot. Hormones are not just for contraception). What I am saying, is that the numbers tell a simple story: when it comes to blood clot risk, on balance, birth control pills are safer than pregnancy.
Similarly, it was Brianna’s call whether to take those blood thinners or not. But knowledge is power and if Brianna decided not to take blood thinners because she figured since she was not on birth control then she was not at risk for blood clots, then she made a choice based on information that was not accurate. Luckily, she did not have any unfortunate, unintended health effects of that choice.
***Disclaimer: I am a doctor, but I am not your doctor. I’m an OBGYN with a passion for health equity and shared decision-making. My blog is intended to share information about the recommendations and thought process that guide my practice. It is not medical advice and because I am not caring for you, I cannot give you any specific recommendations on what to do with your body. Hopefully this information helps guide a conversation with your pregnancy or gynecology provider about your specific situation so you can make a decision together. If you found this article useful, please consider sharing it with a friend and subscribing for updates. If you have a diagnosis or pregnancy/ladyparts question you would like to read more about, send Dr. A a message or leave a comment below!
****Side note: What’s a person-year? This is a type of statistic used to measure an event that can occur over a long period of time. It’s straightforward to say a blood clot did or did not happen during a pregnancy. For blood clots over a lifetime, you can only say that a blood clot has not happened up until the time you are collecting a data. A person-year is a way to account for both the number of people sharing their information for a research study and the amount of time they share information for. So if I am in study for 1 year, I contribute 1 person-year of data. If I am in the study for 5 years, that is 5 person-years, and so on.
Impact of blood clots on the United States infographic. (2024, May 15). Venous Thromboembolism (Blood Clots). https://www.cdc.gov/blood-clots/toolkit/impact-of-blood-clots.html#:~:text=DVT%20and%20PE%20are%20also%20known%20as%20VTE%20(venous%20thromboembolism).&text=VTE%20affects%20as%20many%20as,another%20episode%20within%2010%20years.
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