What Is a Deep Vein Thrombosis (Blood Clot)?

One calf aches. One ankle looks puffier than the other by the end of the day. It would be easy to blame a pulled muscle — and sometimes that is all it is — but a one-sided swollen, aching leg is also the classic calling card of a deep vein thrombosis, usually shortened to DVT. Put simply, a DVT is a blood clot that forms inside a deep vein, most often in the leg. The short video above, "How deep vein thrombosis (DVT) forms" (about two minutes), walks through that process visually: a clot building up inside a deep leg vein and narrowing the channel blood has to flow through.

Step one: the blood slows down

Clots rarely appear out of nowhere. They tend to start when blood moves more slowly than it should — after a long flight or car journey, during a hospital stay, or while recovering from surgery or an injury that keeps you off your feet. A vein wall that has been irritated or injured, and blood that clots more readily than usual, can add to the risk. Pregnancy, hormonal medicines, cancer and its treatment, smoking, and having had a clot before are among the situations clinicians ask about. Often it is a combination: a long journey on top of a recent operation, for example, rather than a single cause.

Step two: a clot forms and the leg complains

As the clot grows, blood struggles to get past it and backs up in the leg. That is what produces the typical picture: swelling in one leg, an aching or cramping pain that can feel like a pulled muscle, warmth over the area, and skin that looks redder or darker than the other side. Here is the catch, though — a DVT does not always announce itself. Some clots cause no symptoms at all and are only found on a scan done for another reason, which is why a clinician's assessment matters more than how the leg feels alone.

Step three: the part that makes doctors act quickly

The reason DVT is taken seriously is what can happen next. A piece of the clot can break away, travel through the heart, and lodge in the lungs — a pulmonary embolism. A second video on this site, "How Deep Vein Thrombosis develop," follows that journey from leg vein to lung. An embolism can cause sudden breathlessness, chest pain that is worse when you breathe in, a rapid heartbeat, or coughing up blood. Those symptoms need emergency care, not a wait-and-see approach. Clots and rhythm problems can overlap, too — our article on what atrial fibrillation is explains another common setting where unwanted clots form.

Step four: confirming it, then treating it

If your clinician suspects a DVT, the usual next step is an ultrasound scan of the leg veins — painless, and able to show whether blood is flowing freely or a clot is blocking the vein. A blood test that looks for a clot-breakdown product is sometimes used first when the suspicion is low. Older bedside sign tests on their own are not reliable enough to rule a clot in or out. Treatment is built around anticoagulant medicines — often called blood thinners — which stop the clot growing and give your body time to break it down gradually, while lowering the chance of an embolism. You may also be advised to keep the leg moving as you recover and, in some cases, to wear a compression stocking. Do not massage a painful swollen leg or start leftover medicines on your own; get the scan first, because the same symptoms can come from a muscle strain, a skin infection, or a cyst behind the knee.

MedicalVideos.com has published trusted medical videos since 2007. The site was founded by Dr. Mohamed Ibrahim, MD, PhD — physician and researcher.

Educational only — this article is general information about deep vein thrombosis, not medical advice and not a diagnosis. If one leg swells or aches without a clear reason, or you develop sudden breathlessness or chest pain, seek medical care promptly.

Frequently Asked Questions

What does a DVT feel like? Typically an aching or cramping pain in one leg, with swelling, warmth, and sometimes redness or darker skin over the area. Some DVTs cause no symptoms at all, so feeling fine does not completely rule one out.

What causes a deep vein thrombosis? Slowed blood flow — from long journeys, bed rest, surgery, or injury — is the most common starting point. A damaged vein wall and blood that clots more easily can contribute, and often several factors combine.

How is a DVT diagnosed? Usually with an ultrasound scan of the leg veins, which shows whether blood is flowing or a clot is present. A blood test for a clot-breakdown product is sometimes used first when suspicion is low.

Why is a DVT dangerous? Part of the clot can break off and travel to the lungs, causing a pulmonary embolism — sudden breathlessness or chest pain that needs emergency treatment. That risk is why prompt assessment and anticoagulant treatment matter.