A study published Tuesday in the Journal of the American Medical Association found an increase in advanced breast cancer among women under 40. According to the Associated Press, the rise in cancer cases is small, but troubling. Tumors found in young women are often more aggressive and grow unchecked because young women are not screened for cancer as regularly as older women.

Researchers say they need more information about what it causing the rise in rates among women under 40. Dr. Rebecca Johnson, lead author of the study and medical director of a teen and young adult cancer program at Seattle Children’s Hospital, told the AP that it's likely that multiple factors are to blame.

“The change might be due to some sort of modifiable risk factor, like a lifestyle change," said Johnson, or exposure to a hazardous chemical.

The study examined the rates of breast cancer among U.S. women ages 25 to 39 between 1976 and 2009, finding that in the mid-1970s an average of 250 young women per year were diagnosed with advanced breast cancer, meaning the disease had progressed to distant parts of the body. In 2009, that number had risen to more than 800. Growth of the U.S. population accounts for some of the increase, Johnson said, "but definitely not all of it."

Overall breast cancer rates have been falling steadily in the U.S., but not as steeply in recent years, indicating that rates may have plateaued. There study showed no increase in advanced cases in women over 40.

Some experts blame higher rates of obesity in America, while others say women's decision to put off parenting until later in life is to blame. Extant tumors exposed to the hormones released in a woman's body during pregnancy can grow rapidly and metastasize to other organs and body systems. Women who have one or more drinks a day have also been shown to have higher rates of breast cancer. Research is ongoing with regards to the effects of cigarette smoking and environmental toxins.

An interagency group empaneled by the authority of 2008′s Breast Cancer and Environmental Research Act issued a report earlier this month rebuking breast cancer researchers for their lack of study into environmental causes of the disease. The report, "Breast Cancer and the Environment — Prioritizing Prevention" said that much more research should be done into the effects of diet, alcohol intake and environmental factors such as exposure to radiation, pesticides, food additives and the dyes and chemicals found in clothing, medications, makeup and other chemical compounds like hairsprays and hand sanitizers.

Johnson, 44, was diagnosed with breast cancer at age 27.

“Young women and their doctors need to understand that it can happen in young women,” she told the AP. When symptoms arise, women should consult their doctors immediately.

“People shouldn’t just watch and wait," she said.

About 1 in 8 women will develop breast cancer in her lifetime, but only 1 in every 173 will develop the disease before age 40. Routine mammograms are recommended for older women, but not for women under 40.

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