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A systematic review of the association between breastfeeding and breast cancer.

Breastfeeding is hypothesized to reduce the risk of breast cancer primarily through two mechanisms, differentiation of breast tissue and reduction of the lifetime number of ovulatory cycles, but previous reviews of the association between breastfeeding and breast cancer have not consistently found that breastfeeding reduces risk of breast cancer. Our systematic review yielded 30 case-control studies and 1 cohort study published between 1999 and 2007. Of the 27 studies that assessed the effect of ever breastfeeding compared with never breastfeeding, only 11 found significant protection against breast cancer. Of the 24 studies of the effect of breastfeeding duration, only 13 found a reduced risk of breast cancer with extended lactation. We conclude that no consensus about the relationship between breastfeeding and breast cancer is emerging. Expanded consideration of possible confounders for this relationship is required to determine if breastfeeding is protective and how protection might be conferred.