Varicose veins can develop through a combination of weakened vein walls and faulty valves. Any superficial vein can become varicose but the most common are those in your legs. Under a variety of circumstances, such as long periods of standing or walking, the increase in pressure in your lower extremities can cause vein walls to become weak.  The recirculating blood that should be moving toward your heart can start to flow backwards.

One-way valves in these veins can also fail to function properly. Normally, these valves open and enable blood to flow through on its way up, then close to stop the blood from flowing back down. If these valves stop working, blood can pool and cause the veins to bulge from excess volume.