Breastfeeding and Breast Cancer: What's the link?


In honor of October and Breast Cancer Awareness month, we are running a mini-series on breast health and the role of breastfeeding. 

Perhaps you've heard that breastfeeding lowers a mother's risk of breast cancer. Well, that sounds pretty good. But how does breastfeeding have any impact on if our bodies develop cancer and why should we care?

Let's do a quick run down of breast cancer first. Breast cancer is the most common cancer for women and as of 2017, the American Cancer Society estimated we'd see around 252,710 new cases of cancer this year in the US alone (1). Not great news. Put very simply, breast cancer is caused when our breast cells go rogue and start dividing and mutating - normal cells become cancer cells. Sometimes this happens in part because of our genetics - breast cancer can certainly run in families. But genetics do not explain the whole picture and, more often, cells in our bodies change during our lives from a variety of causes like age, when we get our first period, whether we have babies or not, to other reasons such as lifestyle choices, what we eat, what we expose our bodies to, if we exercise... you get the idea (3). While the exact causes of this complex disease are not fully known, we do know cell change is key and hormones play a role. 

So what does breastfeeding have to do with cancer?

If you recall from our breast anatomy post, the breasts are organs in the reproductive system that enable mammals to feed their babies. The breast is made for the purpose of making milk and our breasts develop through puberty, pregnancy and even while we lactate with the goal of making sure the system does its job. While pregnancy on its own does seem to be a factor in reducing the risk of breast cancer later in life, it goes hand-in-hand with whether or not we breastfeed AND how long we breastfeed. 


Research seems to highlight two main reasons why breastfeeding may help protect us against breast cancer. First, when we breastfeed, we are encouraging the breasts do their job - produce and eject milk. Essentially, we are telling the cells in our breasts to continue reproducing as breast cells, and this is important! Cells in our body are constantly reproducing, growing, dividing - it's an endless process and these cells can get a memo that tells them to be specialized cells such as cells specifically for the breast or the brain or the liver. They become mature cells in the breast that are involved in the milk making process, and this means they're less likely to mutate into cancerous cells, which are like invaders in the body that don't share the same purpose of the area they overtake (4). Still with me? So, cancer cells in the breast are cells that cannot make milk, they're not specialized for the breast and they cannot contribute to the milk-making process. When we breastfeed, we're constantly giving the cells in our breasts a purpose, essentially preventing them from going off track and becoming cancerous cells. 

The second big reason researchers believe breastfeeding may be important in protecting us against breast cancer comes down to our hormones. Breastfeeding on-demand (and exclusively in the first six months) causes a delay in your period (um, YES!). This is a much welcomed effect of breastfeeding, but it also has a very significant impact on your body. Research has found that the number of ovulation cycles in your lifetime is a risk factor for developing breast cancer, and this could be a result of the high level of hormones like estrogen that your body is exposed to during your cycle (2). Estrogen, for example, is 2-3 times higher during your menstrual cycle and estrogen encourages cells to rapidly reproduce so end up with our body making more cells during our cycles that have the potential to go rogue. When we breastfeed, estrogen is low in our bodies because estrogen can get in the way of your body's ability to make milk (which is why women who breastfeed are recommended to avoid hormonal birth control which contain high levels of estrogen). Low estrogen keeps cell growth in check making it less likely our breast cells will reproduce incorrectly.  Put simply - the less cycles we have, the less exposure we get to estrogen. 

So there you have it! Breast cancer is complex and potentially caused by a whole spectrum of reasons, which makes it difficult to understand and even more challenging to prevent. The good thing is this: research has identified factors that may put us at lower risk. And many of those factors are in our control like making sure we eat well, exercising, not smoking, taking care of ourselves and hey, even breastfeeding! 


  • Breast cancer is a leading issue in women's health and, although we are getting better at treating breast cancer, we need to work on bettering our prevention efforts, which means understanding why women develop breast cancer and what can be done to reduce risk.

  • Having children and breastfeeding lowers the risk of developing breast cancer through a combination of biological and hormonal changes.  

  • The length of time and number of babies you breastfeed may be important too, and we will discuss that in a future post in this mini series, so stay tuned!
  • Breastfeeding does not mean you will never be at risk for developing breast cancer - breastfeeding is great, but we're not claiming that it's magical. It does seem, however, to be an important piece of the puzzle in reducing the risk and THAT is the most important take-away. 


(1) American Cancer Society. (n.d.). Cancer facts and figures 2017. Retrieved from

(2) Clavel-Chapelon, F. & E3N Group (2002). Cumulative number of menstrual cycles and breast cancer risk: results from the E3N cohort study of French women. Cancer Causes Control, 13, 831– 838.

(3) Russo, J., Hu, Y., Yang, X., & Russo, I. (2000). Developmental, cellular, and molecular basis of human breast cancer. J Natl Cancer Inst Monogr, 27, 17-37.

(4) Yang, L. & Jacobsen, K. (2008). A systematic review of the association between breastfeeding and breast cancer. Journal of Women's Health, 17(10), 1635-1645.