A blood clot behind the knee may indicate deep vein thrombosis, a condition in which a blood clot forms in a vein, which sometimes leads to a fatal condition called a pulmonary embolism. According to John Muir Health, a pulmonary embolism causes a sudden blockage in a lung artery due to a clot that breaks free and travels from the leg. See a doctor if a blood clot is suspected.
WebMD states that a deep vein thrombosis may occur anywhere in the body but typically occurs in the leg, thigh or pelvis. Post-surgical patients and long-distance travelers often find themselves at risk of a blood clot. John Muir Health also explains that anyone may develop a blood clot, but people most at risk include those with cardiovascular disease and a family history of DVT. Other risk factors include obesity, sitting for long periods of time and injuries to the vein from major surgery, fractures or severe muscle injury. People with increased estrogen levels due to birth control pills, hormone replacement therapy or having given birth in the past six weeks also have a higher risk of blood clots. WebMD also mentions that an injury that reduces blood flow to part of the body such as a broken hip or leg may also cause a clot.