The incidence of breast cancer in women aged 25 to 39 years old has been rising steadily for 80 years, since before the post-World War II Baby Boom, according to research published in JAMA Open Network and reported in Cancer Network. But, the researchers say, the increase cannot be attributed to parity over time.
“Because our age of interest is younger than 40 years, the significant increase in breast cancer incidence also cannot be explained by increased mammography starting in the 1970s,” the authors wrote. “The trend analysis also shows that the increase in breast cancer began long before routine mammography was initiated.”
In this population-based cohort study, researchers used aggregate-level data from the Connecticut Tumor Registry (CTR), the oldest population-based registry in the US, to look at breast cancer incidence and age-standardized rates among women aged 25-39 years from 1935 to 2015. National mean live births were determined using birth data from the National Vital Statistics System (NVSS), the oldest intergovernmental database for vital statistics, from 1930 to 2015 (allocating for 5-year lag). Moreover, 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.
Of the women in Connecticut aged 25 to 39 years from 1935 to 2015, the incidence of breast cancer increased 0.65% (95% CI, 0.53-0.77) per year, from 16.3 breast cancer diagnoses in 1935 to 38.5 breast cancer diagnoses per 100,000 women in 2015. This increase started almost 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). Further, age-specific parity trends explained only 0% to 4% of the variability in incidence over time.