Breastfeeding and Breast Cancer Risk

Health experts recommend that babies drink only breast milk for their first six months of life, and continue breastfeeding while eating other foods for two years. Most of these reasons have to do with the health of the baby, but breastfeeding also benefits the mom. Notably, it reduces the risk of some types of breast cancer. There’s also emerging evidence that if women who breastfeed do develop tumors, they may be smaller and less aggressive than those who don’t breastfeed.

To fully understand how breastfeeding affects breast cancer risk, Kent Hoskins, MD, says it’s important to know how pregnancy affects breast cancer risk. Hoskins is the director of the Familial Breast Cancer Program at the University of Illinois Cancer Center. 

Pregnancy decreases the risk of hormone receptor-positive breast cancer, but it increases the risk of hormone receptor-negative breast cancer, he says. Breastfeeding seems to balance out the increased risk of receptor-negative subtypes from having a baby. Scientists aren’t entirely sure how breastfeeding alters breast cancer risk, but they do have theories. 

The first theory has to do with how breast cells change during pregnancy and breastfeeding. Breast cells become more mature throughout a person’s lifetime — first during puberty, then if you get pregnant, and then again if you breastfeed. As they become more mature, they’re less likely to become cancerous, Hoskins says. So, because cells become the most mature during lactation, the risk of breast cancer is reduced if you breastfeed.

Another theory is that inflammation may increase the risk of hormone receptor-negative breast cancer. When a person’s breast cells change to be able to lactate after giving birth, but then the person doesn’t breastfeed, those cells rapidly change again. The milk-producing cells die while the others get rearranged. This process causes inflammation, which may increase the risk of hormone receptor-negative subtypes, Hoskins says. When a person breastfeeds and eventually weans, the change in the breast tissue isn’t nearly as rapid or inflammatory.

If breastfeeding does reduce the risk of hormone receptor-positive subtypes, it may be because you don’t menstruate when you’re lactating. Research suggests that the fewer menstruation cycles a person has in their life, the lower their risk of hormone receptor-positive breast cancer. That’s because the hormones estrogen and progesterone rise at different points throughout the cycle, and high levels of these increase the risk of hormone receptor-positive breast cancer. Breastfeeding keeps levels of estrogen and progesterone low.

Additionally, components in the breast milk itself may have anticancer effects, such as immune cells and antibodies that aid the immune system — and may be able to kill potentially cancerous cells in the breast.

Research is clear that breastfeeding reduces the overall risk of breast cancer. But breastfeeding may protect against some types of breast cancer more than others.

Breastfeeding is most protective against hormone receptor-negative breast cancers, which tend to be more aggressive and have fewer treatment options. In one review, one analysis found that breastfeeding reduces the odds of developing hormone receptor-negative breast cancer by 10%, while another found that it reduces the odds by 22%. 

Research has also found that breastfeeding reduces the risk of HER2-positive breast cancer.

More research is needed to understand the effects of breastfeeding on triple-negative breast cancer risk, but the studies that have been done suggest breastfeeding also reduces this type of breast cancer, Hoskins says. 

It’s less clear if breastfeeding reduces the risk of hormone receptor-positive breast cancers. Some research shows that it may reduce the risk a little, and some shows that it doesn’t at all.

Some factors can change how breastfeeding affects your breast cancer risk. Mutations in the BRCA1 gene mostly increase the risk of triple-negative breast cancer. One study found that women with a BRCA1 mutation who breastfeed for more than a year are 22% to 50% less likely than those who never breastfeed to develop breast cancer. The study also found that breastfeeding does not reduce breast cancer risk for women with a BRCA2 mutation, which especially increases the risk of estrogen-receptor positive breast cancer.

Researchers aren’t sure if breastfeeding reduces the risk of breast cancer in breastfeeding transgender men and non-binary people, including those who use testosterone therapy.

“A lot of studies show that you really see the benefit mostly if you’ve done at least 12 months” of breastfeeding, Hoskins says.

But you might not have to do it that long to reduce breast cancer risk. For example, one small study of women with breast cancer found that at least three months of breastfeeding was linked to a later age at diagnosis. This suggests that breastfeeding for at least three months has a protective effect against breast cancer. But due to the study design, it’s unclear if it can stop breast cancer from forming altogether.

Generally, the longer you breastfeed for, the more it reduces the risk of breast cancer. Breastfeeding exclusively for the first six months or longer may reduce breast cancer risk more than also feeding a baby food.

There hasn’t been much research on whether the age you breastfeed at affects how much it reduces your breast cancer risk, Hoskins says. The evidence that does exist is mixed. Some studies find that age doesn’t matter, and some find that not breastfeeding until later in life raises your breast cancer risk compared to breastfeeding at a younger age.

Scientists do know that having a first full-term pregnancy at a younger age reduces the risk of breast cancer compared to doing it at an older age. Those who give birth for the first time when they’re older than 30 have a higher breast cancer risk than those who never give birth. Because of related hormonal effects, breastfeeding at a younger age compared to older age may have similar beneficial effects, Hoskins says.

There hasn’t been much research on whether being breastfed as a baby affects a person’s risk of breast cancer or other cancers. Research suggests that being breastfed for more than three months is linked to a lower risk of blood-related childhood cancer. 

Breastfeeding is good for both mom and baby’s health, but many people find it very difficult to do. There are many resources available online to help you breastfeed, including advice on positions and getting a good latch. If you’re having a lot of trouble, it may help to speak to a lactation consultant.

If you’re struggling with breastfeeding for any reason, you’re not alone. And if breastfeeding isn’t possible or feasible for you, that’s okay. You should do whatever is best for you and your family — and remember that fed is best, even if that’s not with your breast milk.

There are other ways to reduce your risk of breast cancer than breastfeeding. Some actions that greatly reduce your risk include staying active, getting to and staying at a healthy weight, and avoiding or limiting alcohol.