(CBS NEWS) Advanced breast cancer has increased slightly among young women, a 34-year analysis suggests. The disease is still uncommon among women younger than 40, and the small change has experts scratching their heads about possible reasons.

The results are potentially worrisome because young women’s tumors tend to be more aggressive than older women’s, and they’re much less likely to get routine screening for the disease.

Still, that doesn’t explain why there’d be an increase in advanced cases and the researchers and other experts say more work is needed to find answers.

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