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Veins carry blood from your legs, arms, and other parts of your body back to your heart. As your body ages, the valves in your veins weaken, and blood can collect and cause your veins to enlarge and become more visible. This is especially true of veins in the legs and ankles, which have to work against gravity to carry blood up to the heart.  Spider veins affect at least half of adults age 50 and older.  Spider veins are similar to varicose veins, which appear on the legs as people grow older for the same reasons that spider veins do. About half of people who have spider veins have relatives who also have them. This means that while spider veins can be prevented to some degree, in some cases they may be unavoidable. Since the veins of a pregnant woman are under a greater amount of pressure than usual, spider veins may appear, especially on the legs and ankles.  Spider veins caused by pregnancy usually fade a few months after the baby is born. Subsequent pregnancies usually lead to the appearance of more spider veins. Sitting for long periods of time is detrimental to circulation, making it harder for veins in the legs to carry blood up to the heart. This extra strain causes spider veins to appear. Having extra weight puts strain on veins, since the veins must work harder to carry blood to the heart. The veins close to the surface of the face can weaken and become more visible, especially on people with fair skin.
Spider veins are a normal consequence of growing older. Spider veins are hereditary. Pregnancy can lead to spider veins. Having a sedentary lifestyle can lead to spider veins. Obesity can cause spider veins. Sun exposure causes spider veins on the face.