Australian scientists have uncovered why pregnancy and breastfeeding can reduce a woman’s long-term risk of breast cancer.

While it has long been accepted that childbirth and breastfeeding offer protective benefits, previous explanations focused mainly on hormonal changes. Researchers at the Peter MacCallum Cancer Centre in Victoria, led by Professor Sherene Loi, discovered that childbirth generates infection-fighting T-cells in the breasts, which remain in the tissue for decades.

Patients with these T-cells were found to have healthier breast tissue and better outcomes when fighting cancer. “These cells act like local guards, ready to attack abnormal cells that might turn into cancer,” Loi explained. The protection likely evolved to safeguard mothers after childbirth but today also lowers the risk of breast cancer, particularly aggressive triple-negative types.

The study analyzed data from over 1,000 breast cancer patients. Women who had children had significantly more T-cells, and in some groups, they lived longer after a breast cancer diagnosis. Any period of breastfeeding provided benefits, although six months was considered ideal. Loi noted, “When breast cancer cells were introduced, models with this reproductive history were far better at slowing or stopping tumour growth, but only if T-cells were present.”

The research does not claim that pregnancy or breastfeeding guarantees protection against breast cancer. Historical observations, such as the high rates of breast cancer among nuns in the 18th century, first highlighted the link between reproductive history and reduced cancer risk. Previously, pregnancy-related hormones were considered the primary protective factor.

Breast cancer remains a major health concern in Australia, with around 58 women diagnosed daily. It is the most common cancer among women and the second most-diagnosed cancer nationwide.