In middle school your teacher told you they were caused by crossing you legs too much. Though that's not impossible, you're more likely to get these long, treelike lines all over your legs from your DNA. In fact, spider veins, otherwise known as telangiectasias, affect nearly half of adult women in the U.S. "Veins have areas that act as one way-valves to prevent the blood from flowing backward as it moves toward the heart," says Dr. Rob Schwarcz, M.D. "If these valves become weak, blood can back up into the vein and collect there. Pooled blood enlarges the vein and pushes it closer to the surface." Yuck!

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