Breastfeeding and reduced risk of breast cancer in an Icelandic cohort study.

Case-control studies on the association between breastfeeding and the subsequent risk of breast cancer have given inconsistent results. To date, only two cohort studies have been reported on this subject. The present nested case-control study uses data from an Icelandic cohort of 80,219 women visiting a Cancer Detection Clinic that offers population-based cervical and breast cancer screening, in the years 1979-1995. The 993 parous cases were aged 26-90 years at diagnosis, with 9,729 parous controls individually matched on birth year, vital status at case diagnosis, and age when giving information on several potential risk factors for breast cancer. Using conditional logistic regression and confining the analysis to the 84 cases who were <40 years at diagnosis, an inverse association was evident between total duration of breastfeeding and breast cancer, with the adjusted odds ratio = 0.77 per 6 months' increase in duration of breastfeeding (95% confidence interval: 0.59, 1.00), whereas for the remainder of the women, a much weaker trend was observed. Ever lactating was associated with decreased risk, with the adjusted odds ratio = 0.33 (95% confidence interval: 0.19, 0.56) for women diagnosed at all ages. This is the first cohort study to indicate a negative association between breastfeeding and breast cancer.