Breast cancer rates in American women under the age of 40 have been on the rise in recent years. There are known to be racial and ethnic variations within these rates, but are there other demographic differences? A new study finds where you live could impact your risk.

A new study published in the journal Cancer Causes & Control, and supported by the National Cancer Institute, studied early-onset breast cancer trends across all 50 states to see if any area has been disproportionately impacted. 

Dr. Rebecca Kehm, the study’s first author and assistant professor of Epidemiology at Columbia University, says, “Breast cancer incidence is increasing in U.S. women under 40, but until now, it was unknown if incidence trends varied by U.S. geographic region. Our findings can more accurately inform whether exposures that vary in prevalence across the U.S. also contribute to breast cancer risk in younger women.”

The findings, which drew from US Cancer Statistics database figures from 2001 to 2020, showed that 21 states had a case rate increase of more than 0.5% annually in women between the ages of 25 and 39. In the rest of the states, the case rate was stable or went down.

The lowest rate per state was in Wyoming, which had 28.6 cases per 100,000 people, while the highest was in Connecticut, at 41 cases per 100,000 people. The five states with the highest rates were Connecticut, Hawaii, New Jersey, New York, and Maryland.

Certain regions were also particularly impacted. While the West had the lowest incidence rate, it had the highest rate increase. The Northeast, meanwhile, already had the highest overall rate, but it had annual increases in addition to that. The South was the only region whose incidence rate did not go up. The researchers say this is concerning.

Dr. Kehm explains, “The increase in incidence we are seeing is alarming and cannot be explained by genetic factors alone which evolve over much longer periods nor by changes in screening practices given that women under 40 years are below the recommended age for routine mammography screening.”

The study also investigated racial differences, finding that the lowest rates in all regions were in Hispanic women, while the highest rates were in non-Hispanic Black women. Non-Hispanic white women, however, were the only group with a statistically significant increase in all four regions.

The researchers say that further study into whether alcohol consumption may play a role in regional rates is warranted, as it tends to differ across the country and is a breast cancer risk factor. However, the study suggests that geographic region could be a risk factor in itself for early-onset breast cancer, which may allow doctors to understand who may be at higher risk.