A recent study has identified a connection between hormonal intrauterine devices (IUDs) and breast cancer. While this research is significant, media reports suggesting a substantial increase in risk may be causing unnecessary anxiety. Let’s examine the findings in context for those who use IUDs.

What Are IUDs?
IUDs are widely used contraceptive devices designed to prevent pregnancy. They are placed inside the uterus and come in two main types: older versions that contain copper and newer “hormonal” IUDs that gradually release a synthetic form of progesterone called levonorgestrel, mimicking the body’s natural progesterone.

Both the copper and hormonal IUDs are highly effective at preventing pregnancy for many years, and fertility returns quickly after removal. Additionally, hormonal IUDs can make periods lighter and less painful, leading some individuals to choose them even if they do not require contraception.

While many women experience discomfort during insertion or light spotting in the initial months, IUDs are generally well-accepted and commonly used compared to other contraceptives.

What Did the New Study Find?
The recent study conducted by Danish researchers utilized data from national health registries to investigate potential links between hormonal IUD use and breast cancer.

The researchers tracked nearly 80,000 individuals who began using hormonal IUDs over two decades, comparing them to a similar number of individuals born around the same time who did not use these devices. Initially, it might appear that hormonal IUDs prevent breast cancer, as there were 720 cases in the IUD group compared to nearly 900 in the non-IUD group. However, this does not tell the whole story.

In ideal research, a “randomized controlled trial” is conducted, randomly assigning individuals to different treatment groups to ensure they are similar in all respects except for the treatment being tested. This study, however, simply compared individuals who chose to use hormonal IUDs to those who did not, introducing potential confounding factors.

For example, women with higher education levels might be more inclined to opt for IUDs and also more likely to attend breast cancer screenings, leading to differences in detected breast cancer rates.

The researchers adjusted their results for various factors like education, age, number of children, and certain medications and medical conditions. After adjustments, the data suggested a higher risk of breast cancer among hormonal IUD users. However, several critical risk factors for breast cancer—such as body weight, alcohol consumption, smoking, and physical activity—were not accounted for, leaving uncertainty around the findings. Thus, we can only establish an “association” or “link” rather than a definitive cause-and-effect relationship.

How Significant Are the Risks?
Researchers express risk in two ways: “relative” and “absolute” risks. The “relative” risk increase for women using IUDs was about 30% for up to five years, 40% after 5–10 years, and 80% after 10–15 years. While these percentages sound alarming, they don’t indicate the actual proportion of women who will develop breast cancer.

To understand this, we look at “absolute” risk increases, which are much smaller. According to this study, among 10,000 women, there might be an additional 14 cases of breast cancer after up to five years of IUD use, 29 cases after 5–10 years, and 71 cases after 10–15 years. In absolute terms, these increases are all well below 1% of the total number of IUD users.

Highlighting dramatic relative risks without providing the smaller absolute risks is a common pitfall in health risk reporting and does not adhere to best practices in scientific communication.

What Does Other Research Say?
Additional studies on this subject exist, including a large recent study from Sweden involving over half a million hormonal IUD users. This study suggested only a 13% relative risk increase for breast cancer, considerably lower than the findings from the Danish research. This would translate to an additional 1.46 cases of breast cancer for every 10,000 women annually.

This finding aligns with a comprehensive review of related studies, which also indicated a smaller risk compared to the Danish paper. Moreover, the Swedish study examined other cancers, revealing a decreased risk of cervical, ovarian, and endometrial cancers. This mixed risk profile—some increased cancer risk alongside some protective effects—is also observed with traditional contraceptive pills.

Importantly, all forms of contraception help protect women from the risks associated with pregnancy.

What Does This Mean for Me?
The link between hormonal IUDs and breast cancer appears to be minimal and may even be a statistical anomaly. Even if a genuine risk exists, it could be outweighed by protective effects against other cancers. Additionally, this risk may pale in comparison to other breast cancer risk factors, such as high body weight, physical inactivity, alcohol consumption, and smoking. Online resources are available to help visualize these risks.

While hormonal IUDs may not be the right contraceptive option for everyone, they should remain a prominent choice among contraceptive methods.