LOS ANGELES, Dec. 31 (UPI) -- Combining hormones used in hormone-replacement therapy can increase the density of a woman's breast tissue, which can make detecting tumors more difficult, a study released Tuesday suggests.

Researchers at the University of California, Los Angeles, School of Medicine came to that conclusion after studying mammograms of 571 women enrolled in the Postmenopausal Estrogen/Progestin Interventions Trial, a randomized clinical trial investigating the effects of various hormone replacement treatments. Some of the women took only estrogen therapy, some took a combination of estrogen and progestin and some took only a placebo.

As reported in the Jan. 1 issue of the Journal of the National Cancer Institute, after analyzing the data and mammograms, researchers found the estrogen/progestin combination therapy, regardless of how the progestin was given in the treatment, was associated with a 3- to 5-percent increase in breast density. The use of estrogen alone was not linked to any increases in breast density.

"The important aspect of this research is that we see changes in mammograms that reflect what we know about combined estrogen and progestin use and breast cancer risk," researcher and study co-author Dr. Giske Ursin, of the Department of Preventive Medicine at Norris Comprehensive Cancer Center at the University of Southern California, told United Press International.

"The challenge now is to determine whether the women that have the largest mammographic density changes are the ones who go on to develop breast cancer," Ursin said. "Although we know mammographic density in itself is a risk factor for breast cancer, we do not know if a change in density translates to a change in risk."