Spider veins happen when small valves located inside the veins no longer function properly.
In healthy veins, blood is prevented from flowing backwards by these tiny valves. However, if a valve is damaged or weakened, blood can stop moving forward and collect in the vein, causing it to swell and become visible. More severe swelling is called a varicose vein; minor swelling is usually classified as spider veins.
People are more likely to experience spider veins if they:
Are pregnant.
Are overweight.
Are older.
Have a family history of varicose veins.
Work standing or sitting for long periods.