Obese women have a higher risk of breast cancer, and they are usually diagnosed with a later-stage form of the disease. To find out why, researchers from several universities teamed up to determine if mammograms were less effective in overweight women.

What the researchers wanted to know: Obese women have a higher risk of breast cancer and are usually diagnosed with later-stage breast cancer than nonobese women. Are mammograms less accurate on obese women?

What they did: Researchers from several universities analyzed about 100,000 mammograms from a Seattle-based nonprofit health plan.

What they found: Obese women had a 20 percent higher risk of a false-positive mammogram—that is, having a mammogram that incorrectly indicates a woman may have breast cancer. That's a little surprising, the researchers say, because mammograms on fatty breasts are usually easier to interpret than mammograms on dense breasts—and obese women are more likely to have fatty breasts. But the researchers say the higher rate of false positives may be because obese women probably have more breast tissue between the mammography plates, which might scatter more radiation and make the picture harder to read.

What this study means to you: Obese women should know that their chances of a false positive are higher than for women of normal weight.

Caveats: The population in this study was mostly residents of cities in the Northwest and was probably mostly white, so the results don't necessarily apply to everyone. And the calculations of obesity were based on what the women said were their height and weight.

Find out more: Mammography from the Medline Medical Encyclopedia: http://www.nlm.nih.gov/

Read the article: Elmore, J.G. et al. "The Association Between Obesity and Screening Mammography Accuracy." Archives of Internal Medicine. May 2004, Vol. 164, pp. 1140–1147.