I have an IUD. I know many of you do too, because when the recent panic headlines about IUDs and breast cancer appeared, the reaction went to red alert. People were hearing about a new study that linked having an intrauterine device to higher rates of breast cancer. The DMs and inbox filled up, and people started asking me about it in person. At a book event, someone made a beeline for me and opened the conversation with Should I have my IUD out right now? I presume she did not mean literally right there.
Sometimes panic headlines are a nothing-burger, where the underlying research is misleading or low-quality. Sometimes they are cases in which the underlying research is reasonable but the headlines do not follow suit. And sometimes there are cases like this, where the research is reasonable, the headlines do broadly capture it, but the discussion lacks context.
In particular, what is needed here is an understanding of the details of the study, the size of the effects, the broader context of IUDs, and the tradeoffs. That’s a lot to capture in a headline, so it’s fortunate that I get as many words as I want here to ask: what does all the data and information really say?
What does this study show?
In the new study, researchers used large registry-based data from Denmark. As is common in many European countries with national health systems, the researchers have very detailed data on medical treatments and diagnoses and can follow individuals over time.
In this case, researchers identified about 80,000 individuals between the ages of 15 and 49 who started using one of several hormone-based IUDS, and a similar number of control individuals who did not. They excluded individuals who were on other kinds of hormonal birth control or who had had cancer before. Note: this study focused on hormonal IUDs and did not include copper IUDs.
The researchers hypothesized a link between hormonal IUDs and breast cancer in part because we know that other forms of hormonal birth control impact breast cancer. Mechanistically, this may occur because some forms of birth control can stimulate the growth of breast tissue, which could increase the risk of cancer. The question for the paper is whether they see that link with IUDs.
The researchers then use the dataset to follow the individuals over time and look at breast cancer rates. They find that, over any duration of use, there is a 40% increase in breast cancer risk among users of IUDs. As a share, this seems large, but — importantly — the overall numbers are very small. The authors estimate that use of an IUD increases cancer cases by 14 in 10,000 women.
This study has the advantage of being large and having a long follow-up. It also has two significant limitations. First, there is a puzzling fact that the duration of use doesn’t seem to impact the risk. Many of the stories we might tell for why this relationship appears — stories about the role of hormone exposure — would also suggest that more exposure would increase the risk. The fact that we do not see that makes me worry that something else might be driving the relationship. Second, the authors are able to adjust for only a very small number of differences across women. This leads to the concern that other differences across women are driving the effects we see.
The bottom line from this study is that it appears the risk of breast cancer is increased with the use of IUDs. There are legitimate concerns about whether these effects are causal, but this data echoes existing data that suggests that hormonal contraception, including IUDs, can increase breast cancer risk.
This information doesn’t exist in a vacuum. To understand whether it should impact our decisions, we need more context — in particular, we need to think about how this risk compares with other risks, and about whether there are any obvious health tradeoffs.
How large is this risk in comparison?
The effect in the study is small. Given the population and timeline, the authors are focused on cancer that occurs in relatively younger individuals. As noted, their estimate suggests an increase of 14 cases per 10,000 people. This is a 40% increase.
Numbers like this can be difficult to conceptualize: Should we worry about this effect size? Is there some threshold effect we do worry about?
One reasonable approach to this is to compare it with other risk factors. The effect size here is similar to the estimated effect of having a child; women with no children are at higher risk of breast cancer in the data. The effect size is substantially smaller than the increased risk associated with a family history of breast cancer. Weight loss and exercise also have similar size (inverse) effects as IUDs.
In short, there are many lifestyle choices that impact our health risks, including breast cancer risk. In many of these cases (for example, the choice to have children!), a lot of other factors play a role in the decision, as they should. The same is true here.
What about other types of cancer?
The new study focused on breast cancer. But there is substantial existing evidence that IUDs lower the risk of other types of cancer. This includes an estimated 35% reduction in the risk of cervical cancer, a 50% reduction in the risk of endometrial cancer, and a 40% reduction in the risk of ovarian cancer.
These latter effects come from a large study in Finland, run in a very similar way to this new study on breast cancer. And indeed, in the Finnish study, the authors did find an elevated risk of breast cancer. It was not as large as the reduction in the other cancer risks.
Conclusion: Even with a narrow focus on cancer risk, it is not clear that having an IUD increases risk.
What should I do about my IUD?
When we look at a study like this, it is tempting to look at it in isolation. Breast cancer is scary, and, all else being equal, it is clear one would like to lower the risks. If IUDs came in two colors, and the blue ones increased cancer risk and the (otherwise identical) red ones did not, then of course you would choose red.
All else is not equal, however. One direct reason is the other cancer risks. But, taking a broader view, there are clear tradeoffs in choosing not to use an IUD. Some other forms of birth control — the pill, for example — have similarly elevated risks of breast cancer. Condoms carry no risk of cancer, but are not as effective and some people find them less pleasant. Natural birth control methods have a high failure rate. Copper IUDs are an alternative which work for some women, but they often have side effects like heavier menstrual bleeding, making them not a great option for all.
This is a clear case where there is no secret option C. There are risks and benefits to every contraceptive choice. IUDs are a really excellent choice for many women — they provide very effective, long-term contraception with few side effects. They may not be the best choice for women who are at elevated risk for breast cancer due to, for example, a family history, and this is something to discuss with your doctor if you are in this group. If not, I do not think this new study should change what you’re doing.
The bottom line
- A study in Denmark found there was a 40% increase in breast cancer risk among users of IUDs.
- However, the effect size is small, and there is substantial existing evidence that IUDs lower the risk of other types of cancer, including endometrial and ovarian cancer.
- There are risks and benefits to every contraceptive choice, and there is no reason to remove your IUD based on this study alone. Ultimately, this is a decision you should make with your doctor, particularly if you are already at an elevated risk for breast cancer.
Log in
The 40% increase represents 14 in 10,000, suggesting baseline risk of 35 in 10,000? This is much lower than the overall risk of breast cancer in the US, affecting about 1 in 8 women. Does this suggest a very different base risk in the Danish population that does not translate well to countries like the US? Or perhaps it’s because it focuses on such a young cohort?
I actually think the secret option C is paraguard?
I agree that discussing copper IUDs and their potential risks and benefits as part of this topic is worthwhile!
Hi Emily–if you are going to mention “natural birth control,” may I suggest you be more specific? This broad term encompasses a lot of approaches, from the infamous rhythm method to something like the Marquette Method, which has a failure rate that is not all that different from both perfect use and actual use of the pill and condoms. Here’s one source: https://pubmed.ncbi.nlm.nih.gov/18997569
Are there studies showing a similar risk for long-term IVF treatment (because of the hormone injections)?
And does this study test the non-hormonal copper IUD too?
I think this is fair. I also think it’s fair to mention the other issues with IUDs. Many find insertion excruciating. They are inserted incorrectly in some people, not by the fault of the provider, but because it’s an inherent risk. One of my friends lost a “chunk” of her cervix to an IUD (and she was using the non-hormonal kind).
It’s true that plenty of people dislike condoms. But…the issue seems to be about male pleasure, whereas the risks of birth control fall on women. Sigh.
Thank you for this clear and measured assessment! This is definitely scary, as someone who is currently on my second IUD, with some post-menopausal breast cancer family history. Something I personally would add to the discussion of risks/benefits (and wondering if others would as well) is the ENORMOUS health & mental health upside of the side benefits of the IUD I personally have experienced in my 10+ years of use – basically no period, which means no accompanying severe cramps, heavy bleeding, headaches, and mood swings. Yes, cancer is very scary – but my monthly 4-5 days of hell were also pretty miserable. It’s interesting to add some quality of life inputs!
Could copper IUDs be the secret option c? I have not researched this so please excuse if this is a naive question!!
I wondered the same!
This is exactly my question – the discussion seems to focus solely on IUDs that also release hormones. What are the effects of hormone-free IUDs on breast cancer risk?