They are a relatively common condition, and for many people they are a family trait. Women are at least twice as likely as men to develop them. In the U.S. alone, they affect about 23% of all Americans. Many other conditions can predispose people to form varicose veins. The most common are family history, age, female gender, multiple pregnancies, obesity, lack of exercise and career choice. In this disease, the valves are malfunctioning and not working properly. This causes the blood to pool in the vein and makes it difficult for the muscles to push the blood towards the heart. Instead of flowing from one valve to the next, the blood continues to collect in the vein, increasing venous pressure and the likelihood of blockage while causing the vein to enlarge and form varicose veins.