Study Links Pregnancy, Birth and Breastfeeding to Lower Breast Cancer Risk

A new study suggests that pregnancy, childbirth and breastfeeding may help reduce the risk of breast cancer by building up immune T cells in breast tissue. The findings, published on June 14, 2026, indicate that breastfeeding may strengthen the mother’s immune system as well as benefit the baby.

Researchers found that during pregnancy and after delivery, cytotoxic T cells accumulate in the breast. These white blood cells help destroy virus-infected and damaged cells that could turn cancerous, and some remain in the breast for years as immune memory. In tissue samples taken from women who had undergone breast reduction or preventive mastectomy, T cells linked to cancer defense were still present more than 30 years after the last birth among women who had given birth and breastfed for at least six months.

The team also studied mice and found the rise in T cells only in females that went through a full breastfeeding cycle, ending naturally after weaning. Mice that had never given birth, or that stopped nursing unnaturally when the pups were removed, did not show the same increase. Breast tumors in the nursing mice also grew more slowly than tumors in the other groups.

To test the human link, the researchers examined about 1,000 women with aggressive triple-negative breast cancer, all of whom had given birth. Women who had breastfed had more T cells in their tumors, and the amount tended to rise with longer breastfeeding. Their tumors were also more infiltrated by cytotoxic T cells, a feature associated with better recovery odds. The authors caution that the study focused on only one breast cancer type, that pregnancy and breastfeeding involve many overlapping factors, and that ethnicity, the breast microbiome and diet may also affect results. They say the findings could help guide new immunotherapy drugs, more personalized prevention and treatment, and future vaccines or therapies that mimic the T cells' action.