Cancers of the colon and rectum have been declining in older adults in recent decades and have always been considered rare in young people. But scientists are reporting a sharp rise in colorectal cancers in adults as young as their 20s and 30s, an ominous trend.

The vast majority of colorectal cancers are still found in older people, with nearly 90 percent of all cases diagnosed in people over 50. But a new study from the American Cancer Society that analyzed cancer incidence by birth year found that colorectal cancer rates, which had dropped steadily for people born between 1890 and 1950, have been increasing for every generation born since 1950. Experts aren't sure why.

Rectal cancers are rising particularly sharply, far faster than cancers in other parts of the large intestine or colon. The American Cancer Society estimates about 13,500 new cases of colon and rectal cancers will be diagnosed in Americans under 50 this year, with more than 95,500 cases of colon cancer and nearly 40,000 cases of rectal cancer in all age groups.

"People born in 1990, like my son, have double the risk of colon cancer and quadruple the risk of rectal cancer" compared to the risk someone born in 1950 faced at a comparable age, said Rebecca Siegel, an epidemiologist with the American Cancer Society and the lead author of the new report, published in the Journal of the National Cancer Institute on Tuesday. And though the absolute risk is still small in younger people, she said, "They carry the risk forward with them as they age."ontinue reading the main story

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It is the upward trend that is worrisome: The risk of colon cancer for individuals who were born in 1990 was five per million people in that birth group, up from three per million at the same stage of life for those born in 1950. And the risk of rectal cancer for those born in 1990 was four per million, up from 0.9 per million for those born in 1950.

Dr. Thomas Weber, a professor of surgery at SUNY Downstate Medical Center who has served on the National Colorectal Cancer Roundtable and who was not involved in the new study, said the latest research confirms the problem is real and increasing. "There is no mistaking these dramatic increases, especially for rectal cancers," he said, noting that the number of new colorectal cancers among people under 50 each year exceeds the total number of new cases of less common cancers like Hodgkin's lymphoma.

Young people with colorectal cancer run the added risk of a getting a diagnosis later in the course of their disease, when the cancer may be less treatable, because doctors typically don't consider the diagnosis at such a young age. Kirsten Freiborg, who is now 27, complained to doctors repeatedly about having blood in her stool when she was in college, but was told she had internal hemorrhoids. She was finally given a diagnosis of advanced colon cancer a month before her graduation, when she was 22.

Ms. Freiborg's mother, a registered nurse, "kept pushing" for more testing, Ms. Freiborg said, and eventually convinced physicians to send her for a procedure called a flexible sigmoidoscopy, which detected a large tumor in Ms. Freiborg's colon.

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Tara Anderson, 40, of Bowie, Md., was given a diagnosis of colon cancer when she was 38 years old.CreditJustin T. Gellerson for The New York Times

"I still remember getting the phone call from the doctor who did the procedure, who was completely shocked, and said 'I would never have guessed that a 22-year-old would have had cancer,'" said Ms. Freiborg, who was treated with surgery and chemotherapy and is now cancer-free.