Blood clots can happen in veins. A blood clot in a vein close to the skin isn't likely to cause problems. But having blood clots in deep veins is called deep vein thrombosis. Deep vein thrombosis happens most often in the legs. This problem can lead to a blood clot in the lung (pulmonary embolism).The deep veins of the leg have different terms to describe their location:Upper (proximal) leg veins are those above the knee but below the waist. Popliteal veins are located behind the knee.Iliac and femoral veins are in the groin area.Lower (distal) leg veins are those below the knee. Distal leg veins are located in the calf and ankle.A person with a blood clot in a deep upper (proximal) leg vein is at increased risk for a blood clot in the lung or other complications.