SAN FRANCISCO (KCBS) – While cases of colorectal cancer are declining in older adults, a dramatic surge in cases is occurring among Millennials and Generation Xers.

“It’s shocking when you hear about the numbers of people in their 20s and 30s who are being diagnosed,” said Dr. Rebecca Siegel, an epidemiologist with the American Cancer Society and the lead author of this new report.

Siegel said the timing of this colorectal cancer trend parallels that of the obesity trend in the U.S.

“Some of the behaviors that are thought to drive the obesity epidemic, like a more sedentary lifestyle and unhealthy dietary patterns, are also independent risk factors for colorectal cancer,” Siegel told KCBS.

Part of the rise can be attributed to more and better screening, but that tends to apply to people in their 40s.

“The largest increase we see is people in their 20s and 30s, who are least likely to be screened,” Siegel said.

Siegel believes primary care doctors need to be more sensitive to this trend and be on the lookout for red flags. A later diagnosis typically results in a poorer prognosis.