You’ve probably heard of it happening to people on planes: after sitting for a long time without moving their legs, they develop swelling and pain on one side . . . and later find out that they have a dangerous blood clot known as deep vein thrombosis. If caught in time, it can be treated. But often it isn’t, and the clot can break off and travel to the lungs, where it cuts off the oxygen supply and can be lethal.

How does foot swelling play into this? “The swelling is caused by the presence of the clot,” says Dr. Silverstein. “The clot causes pressure to increase behind the area of obstruction, and that increased pressure pushes the fluids in the blood out of the veins into the tissues. “

While anyone who sits for long periods can develop a blood clot, certain factors boost your risk, including obesity, smoking, being pregnant, having heart failure, kidney conditions, a previous clot, cancer, or taking certain medications, such as birth control pills.

Though a clot that’s caused foot swelling doesn’t always have other symptoms, typically you’ll feel pain in your leg as well. “The most common presentation is significant pain and discomfort in the calf region with associated swelling of that limb,” says Dr. Johanning. If you these symptoms strike without explanation, notify your doctor ASAP, or head for the emergency room.

The strategies that prevent your feet from swelling from other reasons can also help prevent blood clots. These include exercising, staying within a healthy body weight range, and taking breaks to move around if you’re sitting for long periods of time.