By on December 13, 2017

Does Hormonal Contraception Increase the Risk of Breast Cancer?

A recent study published in the New England Journal of Medicine examined the association between the use of hormonal contraception (oral contraceptives, hormone-releasing IUD’s, hormonal patches, hormone-releasing vaginal rings) and breast cancer.

The authors looked at data from 1.8 million women ages 15 to 49 in Denmark, followed on average for 10.9 years.  They found that the use of hormonal contraception was associated with a 20% increased risk of breast cancer.  The risk varied based on the type of contraception and duration of use.

As expected, the study received much attention in the media.

So, does hormonal contraception increase the risk of breast cancer?

A few points must be considered:

  1. Although the use of hormonal contraception was associated with an increased risk of breast cancer, that does not mean hormonal contraception caused the increased risk.  In observational studies like this, it is frequently difficult to know if women who used hormonal contraception were at higher risk for breast cancer to begin with.  For example, obese women are more likely to need hormonal contraception to regulate their periods and obese women are also at higher risk of breast cancer.  Or, pregnancy is known to lower the risk of breast cancer and women who use contraception are less likely to get pregnant.  The authors attempted to control for these variables but the data were not complete.  Therefore, it is possible that the increased risk of breast cancer seen in women who used hormonal contraception was due, at least in part, to different risk profiles for breast cancer in the two groups.
  2. The authors found an increased risk of breast cancer in women who used a hormone-releasing IUD, similar in magnitude to women who used oral contraceptives.  The authors concluded that this indicates the hormone-releasing IUD has the same increased risk of breast cancer as an oral contraceptive.  However, it could also mean something else.  It is known that the amount of hormones released into the bloodstream is far less for a hormone-releasing IUD than an oral contraceptive.  So, it is hard to understand how these two contraceptive mechanisms could have a similar effect on the risk of breast cancer.  The fact that the study found a similar magnitude in risk for a hormone-releasing IUD and an oral contraceptive could also indicate that the increased risk is solely due to more risk factors for breast cancer in women who use any form of contraception (as suggested in point #1).
  3. The population studied were all Dutch and had a wide age range (15-49), which may make it less generalizable to other populations.  Also, with the increasing knowledge about genetic predispositions to breast cancer, the role of hormonal contraception would need to be reevaluated and individualized for each woman based on her baseline risk .
  4. Even assuming the data in the study are accurate and that hormonal contraception truly increases the risk of breast cancer by 20%, it is important to realize that this is an increased relative risk of cancer, not and increased absolute risk.  What does this mean?  For example, a 40 year old woman has an estimated 1.5% chance of developing breast cancer in the next 10 years.  A relative increase of 20% would mean her risk would now be 1.8% (not 21.5%).  In other words, 333 women age 40 would need to take hormonal contraceptives for one additional cancer to be diagnosed in the entire group for the next 10 years.  Obviously, every diagnosis of cancer is worrisome, but it is important to recognize that the actual risk of breast cancer is not increased markedly.   The authors themselves noted that 7960 women would have to take hormonal contraception for one year to cause one additional breast cancer.
  5. It is possible that hormonal contraception increases the diagnosis of breast cancer, but not the overall mortality from breast cancer.  It is possible that hormones bring to light an underlying cancer that would be diagnosed later.  Similar data is seen with pregnancy:  there is an increase in breast cancer diagnoses right after pregnancy, but overall, pregnancy reduces the risk of breast cancer.  This implies that the hormones of pregnancy bring out cancers that were already there (but small), but do not actually increase the chance of getting cancer in the first place.
  6. Hormonal contraception has many benefits.  Aside from the obvious, avoiding unwanted pregnancy, it is associated with a lower risk of other cancers (ovarian, endometrial, and colorectal cancer), as well as certain morbidities such as anemia from heavy periods, and painful periods. Also, pregnancy itself is associated with risks such as blood clots, hypertension, bleeding, and surgery. Therefore, any concerns about hormonal contraception due to an increased risk of breast cancer must be weighed against their potential benefits as well.

Ultimately, it is difficult to know if, and by how much, hormonal contraceptives increase the risk of breast cancer.  However, this study does not prove that they do increase the risk of cancer, nor that stopping hormonal contraception would improve the overall health for women.  In fact, the opposite might be true.

Maternal Fetal Medicine blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!