WASHINGTON (AP) -- As controversial as mammograms are for women in their 40s, some get them even younger -- and new research casts doubt on their usefulness.

When to start routine mammograms, at 40 or 50, is debated. But health guidelines don't recommend them before age 40 unless women are at particularly high risk, such as those who carry the BRCA1 and BRCA2 breast cancer-causing genes.

Still, about 29 percent of women in their 30s report having had a mammogram.

So researchers tracked the records of more than 117,000 women who had their first mammogram before 40, and the results support today's guidelines for average-risk women.

If 10,000 35- to 39-year-olds had a routine screening mammogram, 1,266 would be called back for further testing to find 16 with cancer, reported Dr. Bonnie Yankaskas of the University of North Carolina, Chapel Hill. That's less accurate than in older women, and she said those considering a young exam should know the drawbacks: Earlier radiation exposure and extra testing's anxiety and cost.

Her team's study is in Monday's online edition of the Journal of the National Cancer Institute.

The vast majority of these women had no family history of breast cancer, but researchers found no better detection rates in those who did. But the study couldn't track BRCA carriers, an important gap.

Overall accuracy was much better for so-called diagnostic mammograms, done when a woman feels a lump or experiences another symptom that needs checking out.

The study shows better ways to detect breast cancer are needed, plus better evidence to advise BRCA carriers about screening, Dr. Ned Calonge of the Colorado Department of Public Health wrote in an accompanying editorial.

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