Yet the overall rate of cancers in that group is still small. One in 173 women will develop breast cancer before she turns 40, said researchers whose report appeared in the Journal of the American Medical Association - but the prognosis tends to be worse for younger patients.
The study, led by Rebecca Johnson at Seattle Children's Hospital and University of Washington, found that the rate of metastatic breast cancer in particular rose about two percent each year between 1976 and 2009 among younger women.
"We think that the likelihood is that since this change has been so marked over just a couple of decades, that it's something external, a modifiable lifestyle-related risk factor or perhaps an environmental toxic exposure, but we don't know what," Johnson said.
One possibility is that overeating and lack of exercise are driving up early-life metastatic breast cancer rates, Johnson added. Or, the use of hormonal birth control could play a role, she said.
But Johnson also pushed for more research into the potential effects of hormones in meat or plastic in bottles, for example.
Johnson's team analyzed data from cancer registries run by the National Cancer Institute.
The only other change in cancer incidence was among the youngest women, between ages 25 and 39. In that group, the number of women diagnosed with metastatic breast cancer -- which has spread to the bones, brain or lungs -- rose from one in 65,000 in 1976 to one in 34,000 in 2009.
More of the increase appeared to be in cancers that are sensitive to estrogen, which is "comparatively fortunate," the authors note, because those cancers are somewhat more responsive to treatment and have longer average survival rates in general.
Still, metastatic cancer is the most dangers kind, with fewer than one-third of women surviving at least five years after diagnosis, Johnson's team wrote.
But surgeon Julie Margenthaler, who has studied breast cancer in young women at the Washington University School of Medicine in St. Louis, said the new study was limited by a lack of data on women's family history, including which ones were carriers of BRCA gene mutations.
Women with those mutations are known to be at high risk of developing breast and ovarian cancer at a young age.
"It is intriguing data, but I think that it's going to have to be validated in some other datasets," said Margenthaler, who was not involved in the new research.
Both she and Johnson agreed that since the number of women involved is still small, there's no need for the findings to either cause alarm or change standard mammogram practices.
Johnson said women should still be aware that cancer can happen at an early age, even if screening isn't recommended.
"Women need to notice changes in their bodies -- breast lumps, feeling bad -- and promptly seek medical attention for those," she said. "There's a tremendous survival improvement associated with diagnosis before the cancer spreads."