Breast Cancer Incidence Rising Cannot be Attributed to Changes in Parity

Breast cancer incidence in women aged 25 to 39 has been steadily increasing for the last 80 years, though researchers indicated that this increase cannot be attributed to changes in parity over time.

Breast cancer incidence in women aged 25 to 39 has been steadily increasing for the last 80 years, with the increase beginning at least a decade before the baby boom, according to a study published in JAMA Network Open. 

The researchers also suggested that this increase cannot be attributed to changes in 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. 

“Past studies investigating the association of screening with breast cancer incidence have corroborated this finding, concluding that screening accounts for little of the long-term incidence increase, particularly among women younger than 40 years,” the authors wrote. 

Notable limitations within the study include that although the researchers had population-based trend data for cancer and parity information, they were not able to link the 2 data sources; therefore, the study may be confounded by factors associated with the trends in both. Additionally, researchers only used women in Connecticut, which may reduce the generalizability, especially by the racial composition. The researchers were also unable to include data on other risk factors, such as body mass index, mammographic screening rates, or age at first birth. 

According to the study, breast cancer is the most common malignant tumor among women. Moreover, in a 2019 analysis of Surveillance Epidemiology and End Results (SEER) data from 1975 to 2015, researchers 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 and distant-stage disease increased by 2.73% (95% CI, 2.31%-3.14%) per year since 1975. Given that women are generally screened by mammography after 40 years, these APCs cannot be attributed primarily to increased breast cancer screening.



Lima SM, Kehm RD, Swett K, Gonsalves L, Terry MB. Trends in Parity and Breast Cancer Incidence in US Women Younger Than 40 Years From 1935 to 2015. JAMA Network Open. doi:10.1001/jamanetworkopen.2020.0929.