Spider veins happen when circulation becomes compromised or inefficient. Conditions such as pregnancy, standing or sitting for prolonged periods of time or genetic factors can all contribute to their formation. The valves and walls of the veins become weakened due to this inefficient circulation; in response, blood pools up and spider veins form. In the legs, they are related to abnormal blood flow in the affected veins, caused by any number of conditions that puts pressure on your legs, such as weigh gain, sitting or standing for long periods of time and pregnancy. Spider veins on the nose and face are related to overexposure to the sun.