Breastfeeding may reduce the risk of developing breast cancer by up to one fifth, according to researchers presenting the results of a large clinical study at the 2014 San Antonio Breast Cancer Symposium.

Previous studies have shown that breastfeeding has a protective effect against the development of breast cancer, but the benefit appeared to be relatively modest. To better assess the relationship between breastfeeding and the subsequent development of breast cancer, scientists analyzed 27 studies conducted over three decades, involving 750,000 women, 36,000 of whom developed breast cancer.

A meta-analysis of these 27 studies found that breastfeeding reduced the risk of invasive breast cancer by approximately 10%. The study, led by American breast cancer charities in collaboration with Washington University School of Medicine and Mount Sinai Hospital, showed that breastfeeding not only protects against common types of breast cancer but also against more aggressive forms of the disease.

The study did not determine the optimal duration of breastfeeding required to reduce the risk of breast cancer; however, one thing is clear — the longer, the better. Longer periods of breastfeeding may significantly reduce the subsequent risk of breast cancer.