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The number of advanced breast cancer cases among younger woman has tripled in the U.S. over the last three decades, without a corresponding increase in older women, according to a new study.

What's more, the latest findings suggest that this trend may be accelerating, according to researchers.

Researchers said that the incidence of advanced breast cancer in women between the ages of 25 and 30 rose from 1.53 per 100,000 in 1976 to 2.9 cases for every 100,000 women in 2009.  Study authors said that the increase is a small but statistically significant rise in the number of advanced cancer cases in young women, according to the study published Tuesday in the Journal of the American medical Association.

Researchers found that in the U.S. population more than 800 cases of advanced breast cancer occurred in women ages 25 to 30 in 2009 compared to 250 in 1976.

"This study identifies a trend that hasn't been described in the past," the study's lead author Rebecca Johnson, a medical director of the Adolescent and Young Adult oncology program at Seattle Children's Hospital told Bloomberg News. "We'll definitely need future studies to figure out why this change is occurring."

Researchers fear that the trend could be accelerating because the study found that the percent of advanced breast cancer cases increased annually and at a faster rate toward the end of the study.  Researchers said that the increase in cases was independent of race and ethnicity and "shows no evidence for abatement and may indicate increasing epidemiologic and clinical significance," according to a journal news release.

"In the United States, breast cancer is the most common malignant tumor in adolescent and young adult women 15 to 39 years of age, accounting for 14 percent of all cancer in men and women in the age group," researchers wrote in the study.

Researchers found that the individual average risk of a woman developing breast cancer in the U.S. was 1 in 173 by the age of 40 when assessed in 2008.

Researchers said that the trend is worrying because young women with breast cancer generally experience more aggressive disease than older women and have lower survival rates.

According to researchers, the U.S. five0year survival rate for female advanced breast cancer patients between the ages of 20 and 34 is 31 percent compared with 87 percent of women with less aggressive forms of the cancer.

"The trajectory of the incidence trend predicts that an increasing number of young women in the United States will present with metastatic breast cancer in an age group that already has the worst prognosis, no recommended routine screening practice, the least health insurance, and the most potential years of life," researchers wrote.

Researchers analyzed data from the U.S. National Cancer Institute's Surveillance, Epidemiology and End Results registries from 1973-2009, 1992-2009 and 2000-2009 to see whether the breast cancer rate in young women was growing.  They found that only advanced cancers, tumors that spread beyond the breast to other organs, are rising and mostly among women between the ages of 25 and 34 years old.

Researchers noted that progressively smaller increases occurred in older women, and no statistically significant increase in advanced breast cancer rates was seen in women aged 55 or older.

Researchers say that more studies are needed to determine the reason for the increase in advanced breast cancer incidence among younger women.

However, Johnson speculates that the rise could be caused by "toxic environmental exposure or changes in lifestyle over the past 34 years," according to Bloomberg. Other experts believe that the increase may be explained by the fact that women are having children later than women three decades ago.  Past studies have shown that having children early in life is protective against breast cancer.