Spider veins are usually a manifestation of a condition called venous insufficiency. Every vein in the body contains tiny valves that only allow blood to flow toward the heart. If your valves stop functioning properly, blood can flow in the wrong direction and pool within a vein, causing the vein to swell and stop working correctly. When small blood vessels become insufficient, they often turn into spider veins.

There are a number of reasons that veins can become insufficient, including:

Heredity

Female gender

Obesity

Lack of activity

Hormonal changes involved with pregnancy, puberty and menopause

History of blood clots

Use of birth control pills

Prolonged sitting or standing

Along with venous insufficiency, injuries, hormonal changes and sun exposure can also cause spider veins.