Analysis of over 1,000 patient samples and mouse models reveals that pregnancy followed by a complete lactation and weaning cycle establishes lasting CD8+ T cell immunity in breast tissue, specifically protecting against triple-negative breast cancer (TNBC). The protective effect requires the full reproductive sequence—pregnancy alone provides no benefit, and incomplete lactation fails to generate the tissue-resident memory T cells responsible for tumor surveillance. This mechanistic discovery explains decades of epidemiological data showing reduced breast cancer risk in women who have completed breastfeeding cycles. The finding represents a significant advance in understanding how reproductive experiences reshape immune landscapes with lasting consequences. For clinical practice, this suggests timing matters critically—women who breastfeed to completion gain immunological protection that those with shortened lactation periods may not achieve. The research also opens therapeutic avenues, as the CD8+ T cell programming observed could potentially be mimicked through immunological interventions. However, the protection appears most robust against TNBC, the most aggressive subtype, rather than hormone-receptor-positive cancers. This work bridges reproductive biology with cancer immunology in unprecedented detail, offering both preventive insights for individual women and potential targets for developing prophylactic vaccines or treatments that could replicate pregnancy-induced immune programming.