May 15 (HealthDay News) -- Adding to the confusion over exactly what is behind the recent drop in breast cancer incidence rates, government researchers report that mammography screening rates in the United States dropped 4 percent between 2000 and 2005.

The finding raises the possibility that the downward trend might not be as heartening as first thought, because the lower numbers could reflect fewer mammogram screenings and not a real decline in disease incidence. Recent research has also suggested that the fact that fewer women are turning to hormone replacement therapy was the real reason for a 7 percent decrease in breast cancer rates between 2002 and 2003.

"It is concerning, and it needs to be watched," said Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La.

According to background information in the study, which is published in the June 15 issue of Cancer, mammographies are the best way to catch breast cancer early. Since the 1980s, widespread use of mammography has led to a reduction in deaths from breast cancer.

But recent evidence has suggested that the rates of screening are dropping regionally: Thirty-three states have reported a decline in mammography rates among Medicare beneficiaries.

To see if the trend was national in nature, researchers at the National Cancer Institute looked at U.S. mammography data for the years 1987 through 2005.

Between 1987 and 2000, there was a dramatic increase in the use of mammography for women over the age of 40, from 39.1 percent to 70.1 percent. Rates stabilized between 2000 and 2003.

But by 2005, screening rates were 4 percent lower than they were in 2000 (from 70 percent to 66 percent).

In fact, screening rates may be even lower than indicated in this study, because the researchers relied on women's own reporting of whether or not they had been screened, which usually overestimates actual rates.

The declines were most pronounced among women aged 50 to 64 and women with higher incomes, both groups who traditionally have used mammography at higher rates.

It's unclear what caused the decline, although Brooks thinks part of it may be due to "mammogram fatigue."

"When women move into the menopausal years, they tend to lose track of going to a doctor on a yearly basis, and women who got a lot of normal mammograms in their 40s and 50s may ask, 'Why do I continue?' " he said. "But, unfortunately, people get fatigued when the incidence of breast cancer actually rises. The two great risk factors for breast cancer are being a woman and getting older."

The drop might also be due to other factors, such as more women without health insurance, higher co-pays for office visits, and a decline in the number of centers that offer the screening, the study authors pointed out.

What experts do know, however, is that changes in screening rates do translate into changes in the reported incidence of breast cancer and, further down the line, higher death rates.

SOURCES: Jay Brooks, M.D., chairman, hematology/oncology, Ochsner Clinic Foundation, Baton Rouge; June 15, 2007, Cancer