Experts are warning that more and more people younger than 40 years are being diagnosed with colon cancer, saying they could be missing the early signs.

A study released Friday in the medical journal JAMA Network Open said the most common warning sign for the cancer is passing blood in the stool. Abdominal pain, changes in bowel movements and anemia can also be “red flags.”

The paper analyzed 81 studies that examined nearly 25 million adults younger than 50 from various countries across the world.

Nearly half of the individuals in the sample presented with hematochezia — the passage of fresh blood in or with stool — and abdominal pain. One-quarter of individuals noted altered bowl habits.

The experts also explained that delays in detection were common, and the average time from symptom presentation to cancer diagnosis was about six months. Due to a delay in diagnosis, younger adults often present with a more advanced disease by the time it is detected, they noted.

Colon and rectal cancer rates have risen among younger adults but have declined among older adults, who are more likely to get colonoscopies that can catch cancers, the report found.

Millennials born around 1990 are almost twice as likely at risk of colon cancer than people born in the 1950s, The New York Times reported.

Just as younger people may dismiss early warning signs, doctors could too. Anecdotal evidence suggests physicians are less likely to suspect younger people have malignancies, the Times noted.

The analysis did not address the cause of rising rates among colon and rectal cancer in younger adults.

Their findings also come after an expert advisory panel said earlier this year that women should start regular screenings for breast cancer at age 40. The previous guidance said to start at 50.