It’s long been understood that childbirth and breastfeeding can reduce a woman’s risk of developing breast cancer, but the biological mechanism behind this protective effect was traditionally attributed to hormonal changes. Now, groundbreaking research from the Peter MacCallum Cancer Centre in Victoria sheds new light on the subject.

Under the leadership of Professor Sherene Loi, the research team has identified that childbirth stimulates the production of infection-fighting T-cells within the breast tissue. These immune cells remain active in the breast for decades, offering a long-term defense mechanism.

The study revealed that women possessing these T-cells are better equipped to combat cancer, resulting in healthier breast tissue overall. According to Professor Loi, “These cells function as local sentinels, poised to attack any abnormal cells that could develop into cancer.”

She further explained that this protective mechanism might have originally evolved to safeguard mothers during the vulnerable period following pregnancy. In modern times, it significantly reduces the risk of developing breast cancer, particularly the aggressive form known as triple-negative breast cancer.

“This protection may have evolved to defend mothers during the vulnerable post-pregnancy period, but today it also lowers breast cancer risk, especially the aggressive type called triple-negative breast cancer.”

The study used data from over 1000 breast cancer patients.

Of those women, the ones who had children were found to have significantly more T-cells.

And, in some groups, they lived longer after a breast cancer diagnosis.

Any period of breastfeeding provided benefits, however six months was the most ideal, Loi said.

“When breast cancer cells were introduced, the models with this reproductive history were far better at slowing or stopping tumour growth but only if T cells were present,” she added.

It is important to note the research does not claim breastfeeding or childbirth is a guaranteed protection against breast cancer.

Doctors first noticed the link between childbirth and a lower risk of cancer when observing that nuns had some of the highest rates of breast cancer back in the 18th century.

However it was long understood that pregnancy-related hormones were a major factor in preventing cancer.

Around 58 Australians are diagnosed with breast cancer every day.

It is the most common cancer among women and second most-diagnosed cancer in Australia.