(USA TODAY) -- Screening women with both ultrasounds and mammograms allows doctors to find more breast cancers than if they rely on mammograms alone, a new study shows. However, the combination also leads to many more unnecessary biopsies, and experts don't recommend it to most patients.

Researchers involved in a study of more than 2,600 women in today's Journal of the American Medical Association focused on women at high risk, such as those who have had previous breast tumors.

In such women, mammograms found cancer in eight out of 1,000 women screened. Adding an ultrasound let doctors find cancer in 12 out of 1,000, said the study, which was financed by the Avon Foundation and the National Cancer Institute. Together, the tests found 78% of cancers.

But ultra-sensitive tests have drawbacks.

In addition to finding real breast tumors, ultrasounds also pick up many more suspicious spots that are later found to be benign, says radiologist Wendie Berg, the new study's main author. In fact, more than 90% of suspicious ultrasound findings in the study proved harmless.

That can cause women to have needless anxiety and unnecessary biopsies, Berg says.

Because of the risk and expense of additional tests, Berg says most women don't need breast ultrasounds. The American Cancer Society's recommends women begin getting annual mammograms and breast exams at age 40.

Last year, the society recommended that high-risk women combine mammograms with MRI, or magnetic resonance imaging. The society doesn't plan to change that advice, because MRIs are still more accurate, says Debbie Saslow, the group's director of breast and gynecologic cancer.

When combined with a mammogram, MRI finds 93% of breast tumors, Berg's study shows.

But MRI machines aren't available everywhere, Berg says. And she says some women can't tolerate the exams, including those who are allergic to the dye. Ultrasound could be a good alternative for these women, she says.

MRIs are also expensive. Mammograms and ultrasounds cost less than $90 each, but MRIs cost about $1,000, says Shawn Farley, spokesman for the American College of Radiology Imaging Network, which organized the trial.

In an accompanying editorial, Christiane Kuhl of the University of Bonn in Germany notes that ultrasounds are time-consuming. A radiologist would need more than an hour to complete three breast ultrasounds, she writes, but could read 50 mammograms in that time.

And Julie Gralow of the University of Washington cautions that the radiologists in this study were experts in breast imaging. Radiologists with less experience may not perform the tests as reliably, she says.

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