The study findings, published in the Archives of Internal Medicine, found combined hormone therapy -- a combination of estrogen plus progesterone -- was linked to more than one in 10 women having otherwise avoidable mammogram abnormalities and one in 25 women having otherwise avoidable breast biopsies.

"Although breast cancers were significantly increased and were diagnosed at higher stages in the combined hormone group, biopsies in that group less frequently diagnosed cancer, " the study authors said in a statement.

Dr. Rowan T. Chlebowskic of the Los Angeles Biomedical Research Institute at Harbor-University of California, Los Angeles Medical Center and colleagues randomly assigned 8,506 post-menopausal women participating in the Women's Health Initiative clinical trial to receive combined hormone therapy while 8,102 took a placebo.

In the 5.6 years of the study, 199 women in the combined hormone group and 150 women in the placebo group developed breast cancer. Women taking hormones had a 4 percent greater risk of having a mammogram with abnormalities after one year and an 11 percent greater risk after five years, the study said.