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Of 33 cancer types, 14 cancers had increasing rates in at least one younger age group. About 63% of the early-onset cancers were among women.

“These kinds of patterns generally reflect something profound going on,” said Tim Rebbeck of Dana-Farber Cancer Institute, who studies cancer risk and was not involved in the research. “We need to fund research that will help us understand."

The findings were published Thursday in Cancer Discovery, a journal of the American Association for Cancer Research.

How many extra cancers are we talking about?

The researchers compared cancer rates in 2019 to what would be expected based on 2010 rates.

Breast cancer made up the largest share of the excess cancers, with about 4,800 additional cases. There were 2,000 more colorectal cancers compared with what would be expected based on the 2010 rates. There were 1,800 more kidney cancers and 1,200 additional uterine cancers.

Reassuringly, death rates were not rising for most cancers in the young adult age groups, although increasing death rates were seen for colorectal, uterine and testicular cancers.

Why is this happening?

Explanations will take more research. The big databases used for the study don't include information on risk factors or access to care. Theories abound and a big meeting is planned later this year to bring together experts in the area.

“Several of these cancer types are known to be associated with excess body weight and so one of the leading hypotheses is increasing rates of obesity,” said lead author Meredith Shiels of the ��ɫtv Cancer Institute.

Advances in cancer detection and changes in screening guidelines could be behind some early diagnoses.

For breast cancer, the trend toward women having a first child at older ages is a possible explanation. Pregnancy and breastfeeding are known to reduce risk.

It's not a uniform trend for all cancers

This isn't happening across the board. Cancer rates in people under 50 are going down for more than a dozen types of cancer, with the largest declines in lung and prostate cancers.

Cigarette smoking has been declining for decades, which likely accounts for the drop in lung cancer among younger adults.

The drop in prostate cancer is likely tied to updated guidelines discouraging routine PSA testing in younger men because of concerns about overtreatment.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.