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

IMPORTANCE: During the past several decades, breast cancer incidence has been increasing for women younger than 40 years. The increase matches the decrease in parity, an established breast cancer risk factor, but secular trends in incidence have not been examined prior to the 1970s. OBJECTIVE: To examine whether secular trends in parity explain the increase in breast cancer incidence among US women aged 25 to 39 years from 1935 to 2015. DESIGN, SETTING, AND PARTICIPANTS: This population-based cohort study used population-based aggregate-level data from the Connecticut Tumor Registry (CTR) to examine breast cancer incidence and age-standardized rates among women aged 25 to 39 years from 1935 to 2015. National mean live births were calculated using birth data from the National Vital Statistics System (NVSS) from 1930 to 2015 (allowing for 5-year lag). Linear regression was used to compare a baseline model of year estimating age-adjusted breast cancer incidence rate with a model that adjusted for parity constructs. MAIN OUTCOMES AND MEASURES: Breast cancer incidence rates among women aged 25 to 39 years from 1935 to 2015. RESULTS: Among women in Connecticut aged 25 to 39 years from 1935 to 2015, incidence of breast cancer for women aged 25 to 39 years increased 0.65% (95% CI, 0.53%-0.77%) per year, from 16.3 breast cancer diagnoses per 100 000 women in 1935 to 38.5 breast cancer diagnoses per 100 000 women in 2015. This increase began nearly 4 decades before the secular decrease in parity (mean [SD] parity peaked at 2.26 [0.87] live births per woman in 1966 and in 2010 had decreased to 1.41 [0.71] live births per woman). Age-specific parity trends explained only 0% to 4% of the variability in incidence over time. CONCLUSION AND RELEVANCE: These findings suggest that breast cancer incidence for women aged 25 to 39 years has been significantly increasing since the 1930s and cannot be attributed to changes in parity over time.