ATLANTA — The percentage of American women getting mammograms has dropped slightly over the past few years, in what health officials said Thursday is a troubling sign that the battle against breast cancer may be flagging.

The share of women 40 and older who said they had a mammogram in the previous two years slipped from 76.4 percent to 74.6 percent between 2000 and 2005, according to study released by the Centers for Disease Control and Prevention.

The rate had risen dramatically over the past two decades, from 29 percent in 1987, according to American Cancer Society statistics.

The CDC and other researchers said possible explanations for the drop include a shortage of mammography screening centers and specialists, and a lack of health insurance among patients.

Dr. Len Lichtenfeld, the cancer society’s deputy chief medical officer, said the decline may also reflect complacency among women.

“This is a group of women who have ‘grown up’ with mammography as they’ve aged, they’ve perhaps had it done many times over the years and they’ve decided, ‘Well, it’s been OK, maybe I can put it off for a while,”’ he said.

He warned that the recently reported declines in breast cancer incidence rates and cancer deaths are at risk if the decline continues. “If we don’t pay attention now, we run the risk of seeing some of the gains we’ve made reversed,” he said.

The decline of less than 2 percentage points may seem small, but it could be terribly significant, Lichtenfeld said. But if you consider that about 80 million U.S. women should be getting a mammogram every year, it means more than 1 million fewer women are getting the screening test, he said.

And that may mean thousands of cases of breast cancer may not be diagnosed. Women whose breast cancer is caught early have more treatment options and a better chance of beating the disease.

The decline may also at least partly explain a recent drop in U.S. breast cancer rates: It may be that if fewer women are getting mammograms, fewer cases of breast cancer are being discovered.

Some researchers instead tied the drop in breast cancer to reduced use of hormones for menopause.

The study is being published this week in a CDC publication, Morbidity and Mortality Weekly Report. It was based on a national telephone survey of more than 14,000 women in each of the survey years.

The study is not the first to spot the decline. The cancer society keeps statistics, derived from a different national survey, that showed a slight decline in mammogram from 2000 to 2003. Another study of HMO patients showed a decline in screening rates from 1999 to 2002.

Mammography rates increased substantially during the 1990s, so there seems to have been some turning point around 2000.