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.