context: If blood moves too slowly through your veins, it can cause a clump of blood cells called a clot. When a blood clot forms in a vein deep inside your body, it causes what doctors call deep vein thrombosis ( DVT). This is most likely to happen in your lower leg, thigh, or pelvis. But it can occur in other parts of your body, too. DVT can lead to major health problems. In some cases, it can be fatal. That's why if you think you have one, you must see a doctor right away. Not everyone with DVT shows symptoms. But you might notice any of the following: Leg or arm swelling that comes on without warning Pain or soreness when you stand or walk Warmth in the area that hurts Enlarged veins Skin that looks red or blue If a blood clot breaks free and moves through your bloodstream, it can get stuck in a blood vessel of your lung. Doctors call this a pulmonary embolism, or PE. It can be fatal. Some people don't know they have DVT until this happens. Signs of PE include: Shortness of breath Chest pain that's worse when you take a deep breath Coughing up blood Higher heart rate Many things can raise your chances of getting DVT. Here are some of the most common: Age. DVT can happen at any age, but your risk is greater after age 40. Sitting for long periods. When you sit for long stretches of time, the muscles in your lower legs stay lax. This makes it hard for blood to circulate, or move around, the way it should. Long flights or car rides can put you at risk. Bed rest, like when you're in the hospital for a long time, can also keep your muscles still and raise your odds of DVT. Pregnancy . Carrying a baby puts more pressure on the veins in your legs and pelvis. What's more, a clot can happen up to 6 weeks after you give birth. Obesity . People with a body mass index ( BMI) over 30 have a higher chance of DVT. This measures how much body fat you have, compared with your height and weight. Serious health issues. Conditions like Irritable bowel disease, cancer, and heart disease can all raise your risk. Certain inherited blood disorders. Some diseases that run in families can make your blood thicker than normal or cause it to clot more than it should. Injury to a vein. This could result from a broken bone, surgery, or other trauma. Smoking makes blood cells stickier than they should be. It also harms the lining of your blood vessels. This makes it easier for clots to form. Birth control pills or hormone replacement therapy. The estrogen in these raises your blood's ability to clot. ( Progesterone-only pills don't have the same risk.) Your doctor will want to stop the blood clot from getting bigger or breaking off and heading toward your lungs. She'll also want to cut your chances of getting another DVT. This can be done in one of three ways: Medicine. Blood thinners are the most common medications used to treat DVT. They cut your blood's ability to clot. You may need to take them for 6 months. If your symptoms are severe or your clot is very large, your doctor may give you a strong medicine to dissolve it. These medications, called thrombolytics, have serious side effects like sudden bleeding. That's why they're not prescribed very often. Inferior vena cava ( IVC) filter. If you can't take a blood thinner or if one doesn't help, your doctor may insert a small, cone-shaped filter inside your inferior vena cava. That's the largest vein in your body. The filter can catch a large clot before it reaches your lungs. Compression stockings . These special socks are very tight at the ankle and get looser as they reach your knee. This pressure prevents blood from pooling in your veins. You can buy some types at the drugstore. But your doctor may prescribe a stronger version that must be fitted by an expert. Simple lifestyle changes may help lower your odds of getting one. Try these simple tips to keep your blood circulating the way it should: Take care of yourself. Stop smoking, lose weight, and get active. Get regular checkups. And if your doctor has prescribed a medicine to control a health problem, take it as directed. Don't sit for too long. If you're traveling for 4 hours or more, take breaks to flex and stretch your lower leg muscles. If you're on a flight, walk up and down the aisle every half-hour. On long car drives, pull over each hour to stretch. Wear loose-fitting clothes, and drink plenty of water. Plan surgery after-care. Talk to your doctor about what you can do to prevent DVT after surgery. She might suggest you wear compression stockings or take blood thinners. You'll also want to get out of bed and start moving around.