Trends in Parity and Breast Cancer Incidence in US Women Younger Than 40 Years From 1935 to 2015

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Abstract
Breast cancer is the most common malignant tumor among women and has been increasing in incidence for young women in the United States over time.1 In a 2019 analysis of Surveillance Epidemiology and End Results (SEER) data from 1975 to 2015,2 we observed an annual percentage change (APC) of 0.53% (95% CI, 0.29%-0.78%) per year in breast cancer incidence since 1994 for women younger than 40 years; distant-stage disease increased by 2.73% (95% CI, 2.31%-3.14%) per year since 1975. These APCs cannot be attributed primarily to increased breast cancer screening by mammography, as women are generally screened by mammography after age 40 years,3,4 although some guidelines during this period did recommend screening at 35 years and older from 1976 to 1991.5 Using data from the oldest tumor registry in the United States, we investigated whether the APC in breast cancer incidence in women younger than 40 years occurred prior to the 1970s and whether secular changes in national mean parity explain this trend.

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