They are usually found in the thigh and calf and are a common problem. Normally, blood from veins near the surface of the leg passes into the deep veins in the muscle via connecting veins called perforator veins.
Valves prevent blood passing in the opposite direction from deep to surface veins.
If the valves stop working properly the surface veins fill with blood and dilate to become varicose veins. Anyone can get varicose veins. They are usually the result of a poorly functioning valve due to increased back-pressure of the blood in the vein, often as a result of prolonged standing, obesity or hormonal and pressure changes in pregnancy. The main problem with varicose veins is that they are unsightly. They can also cause aching in the calf on standing, which is worse at the end of the day and better when the leg is elevated.