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CHICAGO (AP) -- A new study finds that advanced breast cancer cases have increased slightly among young women. It's still uncommon in women younger than 40, and experts are uncertain about the reasons for the increase.

The results are in an analysis of 34 years of U.S. cancer data. They're potentially worrisome because young women's tumors tend to be more aggressive and they're much less likely to get routine screening than older women.

The study appears in this week's Journal of the American Medical Association.