If your fingernails suddenly start to look swollen, round, or soft, don’t ignore it — your body might be trying to tell you something serious. Doctors call this change “nail clubbing,” and it can be an early warning sign of deadly diseases.

Experts at the Cleveland Clinic explain that clubbing causes your nails to curve downward, becoming wider and spongy. They might also feel warm or appear red. It often starts with the thumb and forefinger before spreading to other nails. “When the inside of the nail begins to dome, it’s time to investigate,” says Dr. Chin Kai Huang from London’s Guy’s and St. Thomas’ Hospital.

This unusual nail change is linked to several serious health issues — including lung cancer, heart disease, liver disease, and gastrointestinal disorders. In fact, studies show that up to 90% of people with lung cancer experience some form of clubbing. Dark lines under the nails can also point to melanoma, a rare but dangerous skin cancer.

If you notice your nails changing shape, don’t wait. See a healthcare professional immediately. As Dr. Huang warns, “Your nails can reveal the truth long before other symptoms appear.” Listening to these small signals could one day save your life.