While breast cancer is still uncommon among women younger than 40, a new analysis shows the number of advanced cases is going up.

And this has left many experts scratching their heads.

Conducted by researchers at the University of Washington, Seattle and St. Charles Health System in Bend, Ore., the study reveals a bizarre anomaly in cancer rates, which could have serious implications for women between the ages of 25 and 39.

The results of the 34-year- analysis 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.

It's likely that the increase has more than one cause, said Dr. Rebecca Johnson, the study's lead author and medical director of a teen and young adult cancer program at Seattle Children's Hospital.

"The change might be due to some sort of modifiable risk factor, like a lifestyle change" or exposure to some sort of cancer-linked substance, she said.

Johnson said the results translate to about 250 advanced cases diagnosed in women younger than 40 in the mid-1970s versus more than 800 in 2009. During those years, the number of women nationwide in that age range went from about 22 million to closer to 30 million - an increase that explains part of the study trend "but definitely not all of it," Johnson said.

Other experts said women delaying pregnancy might be a factor, partly because getting pregnant at an older age might cause an already growing tumor to spread more quickly in response to pregnancy hormones.

Obesity and having at least a drink or two daily have both been linked with breast cancer but research is inconclusive on other possible risk factors, including tobacco and chemicals in the environment. Whether any of these explains the slight increase in advanced disease in young women is unknown.

There was no increase in cancer at other stages in young women. There also was no increase in advanced disease among women older than 40.

Overall U.S. breast cancer rates have mostly fallen in more recent years, although there are signs they may have come to a halt.

About one in 8 women will develop breast cancer in their lifetime, but only 1 in 173 will develop it by age 40. Risks increase with age and certain gene variations can raise the odds.

Routine screening with mammograms is recommended for older women but not those younger than 40.

Dr. Len Lichtenfeld, the American Cancer Society's deputy chief medical officer, said the results support anecdotal reports but that there's no reason to start screening all younger women since breast cancer is still so uncommon for them.

He said the study "is solid and interesting and certainly does raise questions as to why this is being observed." One of the most likely reasons is probably related to changes in childbearing practices, he said, adding that the trend "is clearly something to be followed."

Software engineer Stephanie Carson discovered a large breast tumor that had already spread to her lungs; that diagnosis in 2003 was a huge shock.

"I was so clueless," she said. "I was just 29 and that was the last thing on my mind."

Carson, who lives near St. Louis, had a mastectomy, chemotherapy, radiation and other treatments and she frequently has to try new drugs to keep the cancer at bay.

Because most breast cancer is diagnosed in early stages, there's a misconception that women are treated, and then get on with their lives, Carson said. She and her husband had to abandon hopes of having children, and she's on medical leave from her job.

"It changed the complete course of my life," she said. "But it's still a good life."

Based on reporting by The Associated Press.

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