Breastfeeding does not increase the risk of cancer. Decades of research from global health organizations show the opposite is true: lactation provides significant, measurable protection against certain cancers for the mother. This protective effect is tied to biological changes that occur during and long after nursing. The consensus among scientific bodies is that breastfeeding is a preventative measure, offering reassurance to those concerned about safety. This relationship also extends a protective benefit to the child against specific childhood cancers.
How Breastfeeding Protects Maternal Health
Breastfeeding provides a dose-dependent reduction in the risk of invasive breast cancer for the mother. Research shows that for every 12 months a woman breastfeeds cumulatively over her lifetime, her risk of breast cancer decreases by approximately 4.3%. This protective effect is independent of the risk reduction associated with having given birth. The benefit is particularly notable against aggressive subtypes, such as triple-negative breast cancer.
The total duration of breastfeeding, whether with one child or spread across multiple children, contributes to the overall protection. Any period of breastfeeding offers some benefit, with longer durations providing a greater reduction in risk. Furthermore, breastfeeding has been consistently linked to a lower incidence of ovarian cancer. Women who have breastfed any child show a reduction in their risk of invasive ovarian cancer by about 24% compared to those who have never breastfed.
The duration of lactation also impacts the ovarian cancer benefit, with longer periods offering superior protection. For example, women who breastfeed for seven or more months demonstrate a risk reduction of up to 32%. Remarkably, this protective effect against ovarian cancer is long-lasting, persisting for more than three decades after the last breastfeeding episode.
The Biological Basis for Protection
The primary mechanism for maternal cancer protection involves the profound hormonal changes that occur during lactation. Breastfeeding often suppresses ovulation, which in turn significantly reduces the mother’s lifetime exposure to the cyclical spikes of estrogen and progesterone. Since many common breast cancers are hormone-receptor positive, reducing this exposure limits a factor that can promote tumor growth. This temporary hormonal suppression is a major factor in lowering the risk of hormonally driven cancers.
Physical changes in the breast tissue also contribute to the protective effect by causing terminal differentiation of the cells. During lactation, the cells that line the milk ducts mature fully, making them more specialized and less susceptible to becoming cancerous. The mechanical process of milk production and subsequent involution after weaning also leads to the natural shedding and exfoliation of ductal tissue. This process effectively removes cells that may have accumulated DNA damage or abnormal changes, replacing them with new, healthy cells.
Recent scientific discoveries have also identified a lasting immune component to this protection. Breastfeeding prompts the accumulation of specialized, long-lived CD8+ T-cells within the breast tissue. These immune cells act like resident “local guards” for decades, ready to detect and attack abnormal cells that could potentially develop into cancer. This immunological memory provides a robust, long-term defense, particularly against aggressive forms like triple-negative breast cancer.
Impact on Infant Cancer Risk
The protective benefits of breastfeeding extend beyond the mother and include a lower risk of certain cancers in the child. Numerous studies have consistently shown that infants who are breastfed have a reduced incidence of childhood leukemias. Specifically, breastfeeding for at least six months is associated with a 14% to 20% lower risk of developing acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).
The protective mechanism for the infant is linked to the complex biological composition of breast milk. Breast milk contains a variety of immune factors, including antibodies and proteins like lactoferrin, that support the infant’s developing immune system. These components help establish a healthy gut microbiome, which is understood to be a foundation for proper immune regulation. By providing anti-inflammatory and immune-supporting properties, breast milk may help the infant’s body respond appropriately to early-life infections, a process thought to be related to the development of childhood leukemias.