Spider veins form when a small group of blood vessels close to the skin dilate, or become larger. Also known as telangiectasias or sunburst varicosities, they are red or blue in appearance and can show up anywhere on the body, but most often appear on the face and legs. Although usually medically harmless, they may cause a dull, aching sensation after prolonged standing. In rare cases, they may be associated with more serious health conditions such as liver disease or severe venous disease.   

Veins are an essential part of the cardiovascular system, which transports oxygen and nutrients throughout the body.  The heart pumps blood rich in oxygen and nutrients through arteries to the body. Veins transport depleted blood back to the heart, which pumps the blood through the lungs to pick up fresh oxygen.

Within the veins, valves keep blood moving back toward the heart. When these valves weaken, some of the blood can leak backwards, causing them to become clogged. The congestion will cause the vein to enlarge into a spider vein. Larger dilated veins may become raised above the skin surface. These are called varicose veins and are similar to spider veins. They may be treated in conjunction with nearby spider veins.

Roughly 60 percent of all American women and men will develop some type of vein disorder, according to the U.S. Department of Health and Human Services, but women are more affected. More than 40 percent of women will suffer from abnormal leg veins by the time they are in their 50s.

The age at which spider veins begin to occur varies from patient to patient. They may develop during the teen years in some patients, but in others they may not appear until the person reaches their 40s. According to the American Society of Plastic Surgeons (ASPS), most of the patients seeking treatment for spider veins are between the ages of 30 and 60.