Does breastfeeding lower the risk of breast cancer?

The largest review brought together 47 studies from 30 countries: more than 50,000 women with breast cancer and nearly 97,000 women without it ^[1]. On average, the women with breast cancer had fewer children and had breastfed for a shorter time ^[1].

• With every 12 months of breastfeeding, the risk of breast cancer fell a little; this was separate from the protection that each birth itself gives ^[1].
• The effect was the same in rich and developing countries, in younger and older women, and before and after the menopause ^[1].
• Breastfeeding for all children is added up; the longer a woman breastfeeds in total, the greater the protection ^[1].

In numbers: if women in rich countries had as many children and breastfed as long as was common until recently in developing countries, about 36 fewer of every 1,000 women would develop breast cancer by age 70, that is, less than half as many ^[1]. About two-thirds of this fall would be due to breastfeeding ^[1]. In another review, women who had breastfed for more than 12 months developed breast cancer about a quarter less often ^[2].

Some breast cancers are not sensitive to hormones; these are called "hormone receptor-negative", and one type is "triple-negative". These types are more common in younger women and usually have a worse outlook ^[5]. In a review of 27 studies, breastfeeding was linked to a lower risk of these types ^[5]. See: Treatment of triple-negative breast cancer.

In women who carry a BRCA1 gene mutation, breastfeeding for a year or more was linked to about a third lower risk of breast cancer, and breastfeeding for two years or more gave greater protection; this effect was not seen with a BRCA2 mutation ^[6]. See: Is cancer hereditary?

In a review of 13 studies with nearly 10,000 women with ovarian cancer, women who had breastfed developed this cancer about a quarter less often, separately from the effect of pregnancy itself ^[4].

• Even 1 to 3 months of breastfeeding per child was linked to a lower risk ^[4].
• Breastfeeding for 12 months or more per child gave greater protection: about a third lower risk ^[4].
• The effect was still seen 30 years or more after breastfeeding, and it also applied to the most dangerous type of ovarian cancer ^[4].

See: Symptoms of ovarian cancer and Does the birth control pill cause cancer?

The World Health Organization recommends that babies are fed only breast milk for the first 6 months and that breastfeeding continues up to 2 years of age or beyond ^[7].

• For the mother: besides breast and ovarian cancer, the risk of type 2 diabetes was also lower ^[2]. Breastfeeding also helps with spacing between pregnancies ^[3].
• For the baby: protection against infections, better performance in intelligence tests, and probably less overweight and diabetes later in life ^{[3, 7]}.
• Worldwide: it has been estimated that if almost all mothers breastfed, about 823,000 deaths of children under 5 and 20,000 deaths from breast cancer would be prevented every year ^[3].

Not every mother can breastfeed, or breastfeed for as long as she would like. This is not your fault, and it does not mean you will get cancer. Even in rich countries, where women breastfeed less, about 6 in every 100 women develop breast cancer by age 70; in other words, most women never get it ^[1].

Other steps also lower the risk of breast cancer ^[8]:

• Regular physical activity ^[8]
• Keeping a healthy weight; see: Does obesity cause cancer? ^[8]
• Not drinking alcohol, or drinking less; see: Does alcohol cause cancer? ^[8]

And any amount of breastfeeding, even a few months, is worthwhile ^{[4, 8]}.