Spider veins are a group of dilated veins that appear close to the surface of the skin and look like spider webs or tree branches. They are usually red or blue. Although they can develop anywhere, they are most common on the legs and face. Spider veins are very common. Almost 80 million Americans have spider or varicose veins. Spider and varicose veins are the result of weak or damaged valves in the veins. When the valves do not open to allow blood to leave the veins, blood backs up and the veins swell. A number of factors increase your risk of developing vein problems, including: aging, heredity, history of blood clots, taking birth control pills, hormonal changes during puberty or menopause, pregnancy, standing for long periods of time, wearing tight clothing, obesity, and prolonged sitting with legs crossed.