Breastfeeding and Breast Cancer: A numbers game

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

Welcome back! So in our last post discussing the link of breast cancer and breastfeeding, we discussed how researchers believe breastfeeding triggers various protective responses in our breasts and bodies that can prevent breast cancer. Now we will review where this benefit lies - does it matter if you exclusively breastfeed? Isn't pregnancy and birth enough? What role does the amount of time you nurse play and what happens when you breastfeed more than one baby? What about other factors like race, does that play into the impact breastfeeding has? Let's jump in and sort through all these questions. 



Yes. Research has found the risk for breast cancer decreases 4.3% for every year you breastfeed (1). This number comes from a study that compared over 50,000 cases of breast cancer in 30 countries and is more conservative than other studies that report greater decreases, but we like to err on the conservative side when making connections. And while this number may not be a staggering jump in risk, it does add up quickly. Nursing a toddler? Breastfeeding more than one kid in your lifetime? It all counts. The takeaway - the longer you nurse and the more babies you nurse, the greater your reduction in risk. 


To some extent, maybe. Women who give birth but do not breastfeed are at higher risk for developing breast cancer (2) and, while duration of breastfeeding beyond 12 months shows the most significant protection, breastfeeding at all may help to some degree. A recent meta-analysis (aka when researchers look at a large amount of published studies and analyze them together) showed that having at least one baby and breastfeeding (ever vs never) does reduce the risk of breast cancer (5).


Yes, it matters, too (3). Exclusive breastfeeding for the first six months and breastfeeding on-demand delays your period with greater success than breastfeeding and supplementing and even breastfeeding and bottle-feeding pumped milk because the pump stimulates and drains the breast differently than a baby. As we learned in our last post, breastfeeding and breast cancer are linked due to decreasing the amount of estrogen we're exposed to in our lives because we go months and months (sometimes even years) without our cycles returning. This also means we're going months or years giving our breast cells purpose so they are less likely to go rogue and mutate into cancerous cells -- the longer we breastfeed, the longer this works. Now in order to delay our cycle from returning, we need to be exclusively breastfeeding, which means no other fluids (except medications), limited use of bottle and pacifier, nursing on-demand and having baby sleep in your room (note: this does not mean you have to bedshare, but AAP encourages moms to have baby in the room to respond at night to hunger cues). 


As with so much of healthcare, there are disparities that exist among races and this is important because this means certain groups may be more at risk of developing illness or disease and for these groups breastfeeding is even more important. Let's run through a non-breastfeeding example. If you knew you were more likely to gain weight because it runs in your family, naturally many of us would be more cautious about our diet and lifestyle and take extra precautions or protective steps to reduce the chance of becoming obese right? Now let's look at a breastfeeding example. While the overall risk for breast cancer is similar in non-Hispanic white women and black women, black women have almost double the risk of developing more aggressive types of breast cancer for a variety of reasons like getting their periods earlier in life or the way their body fat is distributed in the body (6). So when research suggests that breastfeeding is biologically protective, it becomes especially important that this community of mamas receives support from healthcare providers, hospitals, loved ones and employers to reach their breastfeeding goals because they have a greater risk (4). Identifying the disparity is helpful, but the lactation community is constantly trying to step it up and address issues like a lack of diversity among lactation consultants among many other factors to reduce racial inequities. If you're interested in reading more about this, please read the Top Five Reasons We Need a Black Breastfeeding Week. 


  • Breastfeeding for 12+ months shows the biggest protection for women but ever breastfeeding versus never breastfeeding may still provide protection. 
  • Each year and each baby you nurse contributes to lowering your risk, which makes sense as we learned that breastfeeding protects against breast cancer in part by delaying the return of your cycle and reducing the amount of estrogen you're exposed to in your lifetime and providing your breast cells with a job (making milk).
  • Breastfeeding may be more important among certain communities depending on genetics or other underlying risk. Sometimes we know we're at risk for breast cancer, but we don't always, so it is important that ALL mothers receive education and support to breastfeed. 
  • Research estimates that the combination of having more than one child and breastfeeding for longer durations beyond a year could reduce breast cancer incidence by more than half and breastfeeding duration is even more important than the number of children you have (1). 
  • Let's reiterate that breastfeeding and breast cancer are complex, and there are still many unknowns in how the two are linked and to what extent breastfeeding plays a role in protecting against breast cancer. We know that breastfeeding is certainly not the only factor in reducing breast cancer risk, other things such as age at first pregnancy and family history play a role as well but it seems that breastfeeding is an important one - and more importantly, it is a factor that, with enough support and resources, we can help moms reach the recommended one year (or more) marker.



(1) Collaborative Group on Hormonal Factors in Breast Cancer. (2002). Breast cancer and breastfeeding: Collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50302 women with breast cancer and 96973 women without the disease. Lancet, 360(9328), 187-195. 

(2) Thunyarat, A. et al. (2013). Risk factors of breast cancer: A systematic review and meta-analysis. Asia-Pacific Journal of Public Health, 25(5), 368-387.

(3) Unar-Munguia, M. et al., (2017). Breastfeeding mode and risk of breast cancer: A dose-response meta-analysis. Journal of Human Lactation, 33(2), 422-434.

(4) Anstey, E. et al. (2017). Breastfeeding and breast cancer risk reduction: Implications for black mothers. American Journal of Preventive Medicine, 53(2S1), S40-S46. 

(5) Babalou, A. (2017). The association of parity and breastfeeding with breast cancer: A review. Health Science Journal, 11(1), 1-5. 

(6) Hall, I. et al.. (2005). Comparative analysis of breast cancer risk factors among African-American women and White women. Am J Epidemiol, 161(1), 40-51.