What causes spider veins?

Any build up of pressure can induce spider veins.

• Blockage from above by bad valves in the deep system of veins
• Crossing legs
• Standing immobile for long periods
• Impact, including, running

...can all produce spider veins. Other factors are inheritance and hormones (including the elevation of estrogens in pregnancy).

When you walk, the muscles that are contracting in your legs "pump" the blood back to your heart. With each squeeze, the blood gets pushed by a one way valve that is supposed to keep the blood from flowing backwards.

For lots of reasons (pregnancy, weight gain, genetics, etc.) the valves become leaky, which allows the blood to flow backwards. The pressure of the blood flowing backwards causes the veins to engorge. The engorged veins can sprout little veins that grow towards the skin. When these veins get near the skin, these veins start to grow, even small veins that grow sideways, underneath the skin. It is these sideways veins that you can easily see as red, blue and purple squiggles that we call spider veins.