Reproductive History and Risk of Second Primary Breast Cancer: The

Background: Women with an initial breast cancer diagnosis are at elevated risk of developing subsequent cancer in the contralateral breast. Studies of reproductive factors and contralateral breast cancer (CBC) have provided inconsistent results. Methods: We employed a case-control study nested within five population-based cancer registries in the United States and Denmark to examine associations between reproductive history and CBC risk. Cases were women with asynchronous CBC who had their first primary invasive breast cancer before age 55 years. Two controls, who had only one primary breast cancer diagnosis, were individually matched to each case on age and year of diagnosis, race, and registry. A total of 694 case-control triplets and 11 case-control pairs wer