Varicose veins may be caused by pregnancy, weight gain, or the individual's body type. Veins are one-way valves that carry blood in one direction. When blood vessel walls become stretched, they get weaker and cause blood to flow in the opposite direction. Blood can also accumulate in the vein and cause them to become swollen and discolored. Due to the fact gravity makes it harder for blood to flow back to the heart, varicose veins usually occur in the legs, as they are furthest from the heart.