A new study conducted by researchers in the United States reveals that thinner women taking hormone replacement therapy could have a higher risk of developing breast cancer compared to heavier women.

Women with a lower body-mass index (BMI) could be more at risk than those with a BMI above 30, which is considered obese, according to the study. Researchers from the University of Southern California studied the data on hormone therapy among almost 3,000 women who were followed for about 10 years, roughly from 1995 to 2006.

They also found that the longer a woman used either estrogen-alone therapy or estrogen-plus-progestin therapy, the higher the risk of breast cancer. The risk was also higher for women who used combined therapy continuously rather than taking breaks during the month.

Using estrogen therapy for 15 years or longer increased the risk of breast cancer by 19 percent, compared with women who had never used HRT. While using combined therapy for over 10 years of time bumped up the risk by 83 percent.

"This gives us a clear correlation between the length of treatment and risk," says Dr. Freya Schnabel, director of breast surgery at NYU Langone Medical Center in New York City. "Ad hoc, people had made the assumption that, for the most part, shorter was better, but this definitely gives you some parameters on that."

Current recommendations on the use of HRT to relieve menopausal symptoms are to use them as little as possible for as short a time as possible, and "this supports the findings and gives a framework for the research," Schnabel says in his report published in the latest issue of the journal Cancer Epidemiology, Biomarkers & Prevention.

A 2002 study published by Women's Health Initiative (WHI) study found that there was dramatic decline in breast cancers as women started reducing the use of HRT.

"There is no question that (the release of information from the WHI) was a cataclysmic moment for HRT in the U.S.," Schnabel says while underscoring the fact that "a lot fewer women are using it, and what we've seen as a result is that breast cancer rates have gone down, and epidemiologists seem pretty comfortable that it was because of that."