(HealthDay News) — The incidence of breast cancer for women aged 25 to 39 years has been increasing since 1935 and seems not to be related to changes in parity over time, according to a study published online March 13 in JAMA Network Open.

Sarah M. Lima, M.P.H., from the Mailman School of Public Health at Columbia University in New York City, and colleagues conducted a population-based cohort study to examine breast cancer incidence and age-standardized rates among women aged 25 to 39 years from 1935 to 2015 using population-based aggregate-level data from the Connecticut Tumor Registry.

The researchers found that from 1935 to 2015, the incidence of breast cancer increased 0.65 percent per year for women aged 25 to 39 years in Connecticut, from 16.3 to 38.5 breast cancer diagnoses per 100,000 women. This increase preceded the secular decrease in parity by nearly four decades (mean parity peaked at 2.26 live births per woman in 1966 and decreased to 1.41 live births per woman in 2010). Only 0 to 4 percent of the variability in incidence over time was explained by age-specific parity trends.

“This study found that breast cancer incidence has been significantly increasing for the past 80 years, with the increase beginning at least a decade before the baby boom,” the authors write. “Findings from our time-series analysis suggest that this increase cannot be explained primarily by secular trends in parity.”

Abstract/Full Text