Spider veins are tiny red, blue, or purple lines on your skin. The veins are flat and usually show up in your legs, but they can also appear elsewhere.
Spider veins usually indicate a problem with the normal functioning of your vein. In some cases, spider veins develop when one of the valves inside your veins doesn’t close properly. This causes blood to leak and become stuck in your vein.
If spider veins aren’t properly treated, they can build up enough pressure to weaken the vein walls. This can allow the spider veins to spread and grow larger.
In some instances, spider veins can turn into varicose veins — twisted, large, and more bulbous-looking veins. Varicose veins might have a rope-like appearance underneath your skin and can feel painful at times, but spider veins don’t usually cause pain or discomfort.