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

Case-control studies on the association between breastfeeding and the subse quent risk of breast cancer have given inconsistent results. To date, only two cohort studies have been reported on this subject. The present nested c ase-control study uses data from an Icelandic cohort of 80,219 women visiti ng a Cancer Detection Clinic;that offers population-based cervical and brea st 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 informat ion on several potential risk factors for breast cancer. Using conditional logistic regression and confining the analysis to the 84 cases who were < 4 0 years at diagnosis, an inverse association was evident between total dura tion of breastfeeding and breast cancer, with the adjusted odds ratio = 0.7 7 per 6 months' increase in duration of breastfeeding (95% confidence inter val: 0.59 1.00), whereas for the remainder of the women, a much weaker tren d 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 negati ve association-between breastfeeding and breast cancer.