Alarmed by an increase in breast cancer diagnoses among younger women and persistently high death rates among Black women in particular, health experts on Tuesday offered a stark revision to the standard medical advice on mammograms.

Women of all racial and ethnic backgrounds who are at average risk for breast cancer should start getting regular mammograms at age 40, instead of treating it as an individual decision until they are 50, as previously recommended, the U.S. Preventive Services Task Force said.

The group issues influential guidelines on preventive health, and its recommendations usually are widely adopted in the United States. But the new advice, issued as a draft, represents something of a reversal.

In 2009, the task force raised the age for starting routine mammograms to 50, from 40. At the time, researchers were concerned that earlier screening would do more harm than good, leading to unnecessary treatment in younger women, including biopsies that turn out to be negative.

But there have been troubling trends in breast cancer in recent years. They include an apparent increase in the number of cancers diagnosed in women under 50 and a failure to narrow the survival gap for younger Black women, who die of breast cancer at twice the rate of white women of the same age.

“We don’t really know why there has been an increase in breast cancer among women in their 40s,” Dr. Carol Mangione, immediate past chair of the task force, said in an interview. “But when more people in a certain age group are getting a condition, then screening of that group is going to be more impactful.”

The new recommendation covers more than 20 million women in the United States between the ages of 40 and 49. In 2019, about 60 percent of women in this age group said they had gotten a mammogram in the past two years, compared with 76 percent of women aged 50 to 64 and 78 percent of women aged 65 to 74.

The panel has said there is insufficient evidence to make recommendations one way or the other for women who were 75 and older.

Dr. Mangione said the task force had for the first time commissioned studies of breast cancer specifically among Black women, as well as for all women, and needed more research into the factors driving the racial disparity. The task force also is calling for a clinical trial to compare the effectiveness of annual and biennial screening among Black women.

Overall, mortality from breast cancer has declined in recent years. Still, it remains the second most common cancer in women after skin cancer and is the second leading cause of cancer deaths, after lung cancer, among women in the United States.

Breast cancer diagnoses among women in their 40s had been increasing at less than 1 percent between 2000 and 2015. But the rate rose by 2 percent a year on average between 2015 and 2019, the task force noted.

The reasons are not entirely clear. Postponement of childbearing, or not having children at all, may be fueling the rise, said Rebecca Siegel, senior scientific director of surveillance research at the American Cancer Society. Having children before age 35 reduces the risk of breast cancer, as does breast feeding.

Still, she noted, there is much year-to-year variation in the diagnosis rates. Other researchers suggest the increase among younger women may simply reflect more screening, said Dr. Steven Woloshin, professor of medicine at Dartmouth University.

Frequent screening can itself cause harm, leading to unnecessary biopsies that cause anxiety and treatment for slow-growing cancers that would never have been life-threatening, researchers have found.

Yet there was a firestorm of criticism in 2009, from both patients and advocacy groups, when the task force advised that women start getting regular mammograms no later than age 50. Critics of that guidance feared that malignancies would be missed among younger women and suggested that a desire to cut health care costs drove the recommendation.

At the time, the panel also called for longer intervals between mammograms: one every two years, rather than annual scans. That recommendation still stands.

The American Cancer Society differs on this key point. Women aged 40 to 44 should be able to choose screening, the society says, but beginning at 45, women should get mammograms every year until age 55, when the risk of breast cancer begins to drop.

Karen E. Knudsen, chief executive officer of the cancer society, said she welcomed the task force’s advice to begin routine screening at a younger age because it will alleviate confusion resulting from contradictory recommendations from medical groups.

Still, she said, “We are steadfast on annual screening. Cancers in premenopausal women grow faster, and it’s important they don’t develop during the two-year period and go undetected.”

The task force’s new recommendation applies to all people assigned female at birth who are asymptomatic and at average risk for breast cancer, including those with dense breast tissue and a family history of breast cancer.

But the advice does not apply to anyone who already has had breast cancer, carries genetic mutations that increase her risk, has had breast lesions identified in previous biopsies or has had high-dose radiation to the chest, which raises the risk of cancer.

These women should consult with their doctors about how frequently to be screened.

The task force emphasized that it was important for Black women to start mammograms at age 40, as they are more likely to get aggressive tumors at a young age and 40 percent more likely to die from breast cancer than white women are.

Some scientists have called for moving away from a universal, one-size-fits-all approach to screening in favor of a “risk-adapted” approach, which would mean screening Black women six to eight years earlier than white women.

“The recommendation should be tailored by race and ethnicity to maximize the benefits of screening and minimize its harms and to address the current racial disparity,” said Dr. Mahdi Fallah, who studies risk-adapted cancer prevention at the German Cancer Research Center in Heidelberg.

But screening alone will not improve survival rates for Black women, who not only are more likely to develop aggressive tumors but also to struggle with delays getting medical care and with life circumstances that make treatment difficult.

The task force’s new report found, for example, that while follow-up of abnormal breast scans is often delayed, it’s especially true for Black women.

“So often when it’s a Black woman, you hear a narrative you wish you weren’t hearing,” Dr. Mangione said.

“Oftentimes, these are women who find a lump themselves, or a discharge they know is abnormal, and they go in and they get dismissed. And it’s only because they’re not willing to accept no for an answer that they are ultimately diagnosed.”