CHICAGO (February 26, 2013)--A new study found that advanced breast cancer cases have increased slightly among young women and.

Advanced breast cancer is uncommon in women younger than 40, and experts are uncertain about the reasons for the increase.

The findings come from 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 appeared in this week's Journal of the American Medical Association.