The exact cause of spider veins is unknown, although heredity is believed to play a strong role. Research has shown that identical twins sometimes will have spider veins in the same areas of the body and to a similar extent, due to inherited weak vein valves.

Pregnancy and hormonal influences also contribute to the development of spider veins. During pregnancy, the volume of blood in the body increases greatly, but the blood flow decreases from the legs to the pelvis causing veins to swell. Hormonal changes during puberty, pregnancy and menopause also appear to contribute to enlarged veins. Internal disease is rarely the cause of spider veins.

Women are more at risk for spider veins than men. In addition to pregnancy and hormonal fluctuations, other factors that may contribute to spider veins include birth control pills and hormone replacement therapies. Tight girdles or hosiery held up with tight elastic bands also can cause spider veins.

Other factors known to contribute to spider veins include:

• Aging. As a person ages, spider veins are more likely to occur.
  
  
• Leg injury. Injuries to the leg can damage the veins, causing spider veins.
  
  
• Sedentary lifestyle. Not exercising weakens muscles and impairs good blood circulation and healthy veins.
  
  
• Long periods of sitting or standing. This can cause blood to pool, enlarging veins.
  
  
• Obesity. Excess weight increases pressure on the veins.
  
  
• Sun exposure. People with light colored skin are susceptible to spider veins on the nose or cheeks that may be caused by sun exposure.