This study found no significant overall association relative to women who had never breastfed - after adjusting for established risk factors for breast cancer - between a history of having breastfed and subsequent development of breast cancer (relative risk [RR] 0.93, 95% CI 0.83 -1.03).
No inverse trend was observed with duration of breastfeeding; women who breastfed for 2 years of longer had a RR of 1.11 (0.90-1.38). Among women who had given birth only once, women who had breastfed their child experienced a lower incidence of breast cancer (RR 0.68, 0.46-1.00). Among premenopausal women, who tended to be near menopause due to the age structure of the cohort, the RR of breast cancer for those who had lactated was 1.16 (0.89-1.50). Premenopausal women who had lactated for 1 year or more had a RR of 1.10 (0.78-1.57).
The authors state that these data suggest that there is no important overall association between breastfeeding and the occurrence of breast cancer.
Michels KB et al (1996) Prospective assessment of breastfeeding and breast cancer incidence among 89,887 women. Lancet 347: 431-6 [Abstract] (this study found no reduced risk)