In a groundbreaking discovery, Australian scientists have uncovered compelling evidence that pregnancy and breastfeeding may help the body build long-term immune defenses against breast cancer—particularly the aggressive triple-negative form. The research sheds new light on centuries-old observations linking childbearing to reduced cancer risk and offers biological explanations for the phenomenon.
For generations, medical observers noted that women who did not bear children—such as nuns in 18th-century Europe—experienced higher rates of breast cancer. While early theories attributed this to hormonal differences, new research suggests the real key lies in the immune system’s memory.
According to Professor Sherene Loi, a leading oncologist and the study’s senior author, pregnancy and breastfeeding appear to leave behind “long-lived protective immune cells” in the breast and throughout the body. These specialized T cells act as sentinels, detecting and eliminating abnormal cells that could evolve into cancer. “They help to reduce risk and improve defense against breast cancer, particularly triple-negative breast cancer,” Loi told ABC News Australia.
Triple-negative breast cancer lacks the three major receptors that typically guide targeted treatment, making it more difficult to treat and often more lethal. Although it accounts for a smaller share of total breast cancer cases, it disproportionately affects younger women.
The findings, published in Nature, revealed that women who had given birth and breastfed had notably higher concentrations of T cells within their breast tissue—cells similar to those used in advanced cancer therapies such as CAR-T treatment. Remarkably, these immune cells persisted for years after breastfeeding ended, suggesting a lasting protective effect.
Animal experiments reinforced the human data. When cancerous cells were introduced into mice that had never reproduced, were actively nursing, or had previously nursed, the latter two groups exhibited slower tumor growth and higher T-cell activity. Once researchers removed the immune cells, the tumors began to grow unchecked—demonstrating their crucial defensive role.
A broader analysis of data from more than 1,000 women with triple-negative breast cancer further confirmed that those who had breastfed tended to fare better, with tumors showing higher immune activity.
While the protective effect varies between individuals, researchers estimate that each pregnancy reduces breast cancer risk by about 7%, while every five months of breastfeeding cuts the risk by an additional 2%. Given that roughly one in eight women globally will face breast cancer in their lifetime, these figures represent a significant preventive benefit.
Professor Loi also emphasized that lifestyle changes in modern societies—such as delaying childbirth or choosing not to breastfeed—could partially explain rising cancer rates. “Our findings highlight how profoundly the immune system is shaped by reproduction,” she said. “It’s a reminder that some of our oldest biological processes continue to protect us in ways we’re only just beginning to understand.”