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

A new study suggests that pregnancy, childbirth and breastfeeding may help lower the risk of breast cancer by boosting the mother’s immune defenses in the breast. The article, published on June 14, 2026, says breastfeeding appears to strengthen the immune system’s ability to block tumors, especially through the buildup of T cells in breast tissue.

The researchers found that during pregnancy, the breast undergoes changes that prepare it for milk production, and cytotoxic T cells accumulate in the tissue. These white blood cells kill virus-infected and damaged cells that could turn cancerous, and some remain in the breast long term as immune memory. In samples taken from women who had breast reduction surgery or preventive mastectomy, T cells specialized in removing cancer cells were still present more than 30 years after their last birth in women who had given birth and breastfed for at least six months.

The team also studied mice and found that only females that completed a full breastfeeding cycle, ending with natural weaning, showed a significant rise in T cells in the mammary glands. Females that never gave birth, or that stopped nursing unnaturally after their pups were removed, did not show the same increase. Consistent with that, breast tumors in the nipple area of nursing mice 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 breastfed had more T cells inside their tumors, and the amount increased with longer breastfeeding. Their tumors were also more infiltrated by cytotoxic T cells, a feature associated with better recovery odds. The study’s limits include its focus on only one breast cancer subtype, and the difficulty of separating the effects of pregnancy, childbirth and breastfeeding, along with possible differences by ethnicity, microbiome and diet. Even so, the findings could help guide new immunotherapy drugs, personalized prevention and treatment approaches, and future studies that take pregnancy and breastfeeding history into account.