U.S. women are getting mammograms to screen for breast cancer at declining rates, according to a study describing a trend that experts fear may portend a reversal of progress against the deadly disease.

The percentage of women 40 and older saying they had a mammogram within the past two years slipped from 70 percent in 2000 to 66 percent in 2005, according to the study appearing on Monday in the journal Cancer.

This upends big increases since the 1980s. The mammography rate for women past the age of 40 was only 39 percent in 1987.

Researchers led by Dr. Nancy Breen of the National Cancer Institute said the findings followed previous indications from various parts of the country that the popularity of mammograms was ebbing.

“It’s quite an unusual and disconcerting finding,” Breen said in a telephone interview. “It comes as a surprise because there’s no reason you’d think there’d be a drop in mammography.”

But other experts point to a number of possible reasons, including insurance issues and recent doubts cast on the benefits of mammograms - breast X-rays that women notoriously dislike because of the discomfort involved.

Most studies show that widespread use of mammograms has made early detection of breast cancer more common and reduced death rates from the disease.

Mammograms are used to screen healthy women for signs of breast cancer and are considered a crucial tool to detect the disease at its earliest stages when it is most treatable.

The study found declines among groups who traditionally have used mammography at high rates, including higher-income and better-educated women, those in ages 50 to 64 and non-Hispanic whites.

The findings were based on a scientific survey of about 10,000 U.S. women 40 and older by the Centers for Disease Control and Prevention, Breen said. Mammography rates stagnated between 2000 and 2003 and then dropped in 2005, she said.

DEATH RATES

Robert Smith, the American Cancer Society’s director of cancer screening, said the trend could lead to more breast cancer deaths.

“A decline in mammography utilization is going to result in a higher rate of cancers diagnosed at an advanced stage,” he said in a telephone interview. “And that will mean more aggressive treatment, and in some instances it may mean that women who would have survived if their cancer had been found earlier will not survive.”

The society recommends annual mammograms for all women starting at age 40. The National Cancer Institute recommends them for these women every one to two years. Experts also recommend mammograms for younger women with symptoms of breast cancer or who are at high risk for it.

The researchers said the drop in mammography rates may be caused by a number of factors, including an increase in the number of women without health insurance and less emphasis on mammography in health-promotion campaigns.

Other potential factors they cited were a 2002 controversy that may have undermined some women’s confidence in mammograms after experts like Donald Berry of the M.D. Anderson Cancer Center in Houston questioned their value, and less vigilance toward breast cancer due to the dropping death rates.

Breast cancer is the second leading cause of cancer death in women behind only lung cancer. In the United States, an estimated 178,000 women will be diagnosed with breast cancer this year and about 40,000 will die from it.

“The looming questions are whether the decline in mammography will continue and how it will affect mortality rates from breast cancer,” the researchers wrote.