Nearly a million Americans develop blood clots each year, making it incredibly important to recognize the early warning signs. Blood clots—also known as thrombosis—can be life-threatening if they aren’t treated quickly. Below is a breakdown of how clots feel, what symptoms to look out for, and when to get medical help.

1. What Exactly Is a Blood Clot?

A blood clot is essentially a clump of blood that forms in your veins or arteries. While these clots are useful when stopping bleeding after an injury, they can turn dangerous if they develop without an obvious cause. “When blood clots form in the deeper system, they can be painful and very dangerous,” says Dr. Luis Navarro, founder of the Vein Treatment Center in New York City. These dangerous clots—called deep vein thrombosis or DVT—can restrict blood flow and lead to serious health issues.

2. Swelling in One Limb

One of the most noticeable signs of DVT is swelling in just one arm or leg. “Blood clots can block the healthy flow of blood in the legs, and blood can pool behind the clot causing swelling,” explains Dr. Navarro. According to the CDC, signs of a clot in your leg include swelling, redness, and tenderness. If you experience sudden swelling—especially if it’s painful—don’t ignore it. Call a doctor.

3. Pain in the Leg or Arm