This is a circulation problem stemming from incompetent valves in the vessels. These valves are supposed to prevent the backflow of blood, helping it move back up toward the brain - no easy feat considering the distance from your legs to your head and the downward pull of gravity. When they're working properly, the valves prevent blood from rushing down to the feet (another brilliant engineering facet of the body). People with varicose veins have "floppy valves" that don't open and close effectively, which causes blood to flood these leg vessels. The condition definitely worsens in pregnancy, perhaps because the blood flow doubles at that time.
This circulatory condition is partially genetic, but it can occur as a result of physical trauma too. I got my spider veins and varicose veins after a bad bicycle accident compressed my leg. In my situation, I think genetics has played a part too, and my varicose veins did get worse during my pregnancies.
More Answers from Ellen Marmur, MD