A new Swedish prospective study presented at the seventh European Breast Cancer Conference (EBCC7) in Spain has reported that longer duration of breastfeeding is associated with more unfavorable breast cancer characteristics. The study included 17,035 women in the Malmö Diet and Cancer study. During an average follow-up period of 10.2 years, 622 new breast cancers were diagnosed. Tumors were classified according to invasiveness, tumor size, axillary lymph node status, and hormone receptor status, among other characteristics. Duration of breastfeeding was defined as average period of breastfeeding per child and divided into quartiles (under 2.2 months, 2.2 to just under 4.0 months, 4.0 to just under 6.2 months, and at least 6.2 months). The lowest quartile was used as the comparison group.

Overall risk of breast cancer was found to be comparable for all four quartiles of breastfeeding duration. However, among women with the longest duration (at least 6.2 months), there was a statistically significant higher risk of grade III tumors and tumors with high Ki-67 (a cancer antigen that is found in growing, dividing cells and whose presence indicates less favorable prognosis). Results were similar when total duration of breastfeeding and duration of breastfeeding of the first child were examined instead of average time of breastfeeding per child. The authors conclude that long duration of breastfeeding is associated with more unfavorable breast tumor characteristics than short duration.

Comments concerning the study

Breastfeeding has been found in multiple studies to reduce the risk of breast cancer. It has been assumed that longer periods of breastfeeding are overall more beneficial than shorter periods (note that this study did not compare women who breastfed after giving birth with those who did not). The study results call for an explanation of the underlying mechanism of action. It is possible that longer duration of breastfeeding prevented most of the lower grade tumors that might otherwise have occurred in the study participants, leaving the most aggressive tumors. On the other hand, the remarkable hormonal and cellular changes involved in milk production might have increased the likelihood of aggressive tumors in some of the women over time.