Roughly two million people in the US each year are affected by deep vein thrombosis (DVT)—blood clots (thrombi) that develop in the deep veins of the legs or, less commonly, the pelvis or other parts of the body.

A blood clot may sound harmless—after all, clotting is a natural process that helps stop bleeding after an injury. However, clots can also form if blood flow is too slow, there is damage to the lining of the veins, or if a medical problem such as an inherited clotting disorder causes the blood to clot more easily. If a clot breaks loose and travels to the lungs, a complication called pulmonary embolism can occur. Embolisms disrupt blood flow to the lungs and can be fatal.

Anyone can develop a dangerous blood clot. Your risk may increase:

Following surgery. Blood clots are the leading cause of disability and death following surgical procedures. Patients may be instructed to take medication before or after surgery, wear special stockings or boots that help squeeze the muscles that keep blood flowing, or elevate their feet.

During pregnancy, changes in the blood and increased pressure in the deep veins of the legs up a woman’s DVT risk 10-fold. The risk remains high after giving birth. Pregnant women with a history of DVT, inherited clotting disorders, and those who will need to be on bed rest or who may have a cesarean delivery may need treatment in advance to avoid clots.

While taking hormone medications. The use of birth control pills or hormone therapy raises the risk of DVT, especially in older women or women who smoke.

While taking a long trip. Sitting for four hours or more, such as during long-distance travel, can double your chances of DVT. When traveling, drink lots of fluids, wear loose-fitting clothing, and build in breaks to walk and stretch at regular intervals.

Roughly 50% of people with DVT in the calf, leg, or ankle experience symptoms in the affected area which may include warmth or tenderness, pain or sudden swelling, redness of the skin, or constant pain in one leg while standing or walking.

Call 911 immediately if you experience the signs of a pulmonary embolism such as a sudden cough (which may produce blood); sudden shortness of breath; pain in the ribs when breathing; sharp chest pain under the breast or on one side; burning, aching, or dull heavy feeling in the chest; rapid breathing; or rapid heart rate.