Breast cancer death rates for U.S. women have been falling for decades.

Now, that trend has ended for younger women. New research published in the journal Radiology shows that breast cancer death rates for women under 40 are no longer decreasing.

The study looked at data gathered by the National Center for Health Statistics between 1969 and 2017. The breast cancer mortality rate had been falling steadily between 1989and 2010. It decreased between 1.5 and 3.4 percent per year for each decade of age for adult women from 20 to 79, as breast cancer screening rates steadily rose.

The trend has continued to hold for women 40 and older. But beginning in 2010, the breast cancer death rate rose 2.8 percent per year in women in their 20s and 0.3 percent per year in women in their 30s.

The researchers attribute the rise to a “significant” increase in Stage 4 breast cancer among younger women. The number of 25- to 39-year-olds with metastatic breast cancer increased 32 percent between 2009 and 2015.

The death rate decline also slowed for women in their 70s, who have also seen a rise in Stage 4 breast cancer.

Why do breast cancer deaths rates continue to fall for women in their 40s, 50s and 60s? The researchers think regular screening plays a critical role. But younger women, who are not regularly screened unless they are at a high risk for breast cancer, may therefore have cancers that go undetected.

Breast cancer screening guidelines have shifted in recent years. The current recommendation is that women between 50 and 74 get a mammogram every two years, and that younger women talk to their doctors about when to begin screening.