Arteries bring oxygvenen-rich blood from your heart to the rest of your body and veins return oxygen-poor blood back to your heart. You have three kinds of veins in your legs; the superficial veins, which lie closest to your skin, the deep veins, which lie in groups of muscles, and  perforating veins, which connect the superficial veins to the deep veins. The deep veins lead to the vena cava, your body's largest vein, which runs directly to your heart. The major superficial vein in the leg is the great saphenous vein. This vein, also called the GSV, has many  smaller branches coming off of it, like a tree. When these veins become enlarged, they become varicose veins.

When you are in the upright position, the blood in your leg veins, including the GSV, must work against gravity to return to your heart. One-way flaps, called valves, in your veins keep blood flowing in the right direction. When your leg muscles contract, the valves inside your veins  open. When your legs relax, the valves close. This prevents blood from flowing in reverse, back down the legs. The entire process of sending blood back to the heart is called the venous pump.

If the valves inside your veins are not working properly, they may allow the blood  to flow backwards, towards the feet. Sometimes these valves are called “leaky” valves.

The medical term for backward blood flow in the leg veins is venous reflux disease. When venous reflux disease  occurs in the GSV, the blood will pool inside the superficial veins, which may cause those veins to become swollen and enlarged, and appear as unsightly varicose veins.

Factors that can increase your risk for varicose veins include having a family history of varicose veins, having had multiple pregnancies, older age, being overweight, standing or sitting for long periods of time, or having had a previous blood clot in the leg veins. Women are more  likely than men to develop varicose veins. 

Pregnancy in particular can increase your risk for developing varicose veins. During pregnancy, blood volume increases, distending the veins. As the pregnancy progresses and the stomach gets bigger, the legs must carry extra weight, compressing the veins. Varicose veins may  improve a few months after the pregnancy is over, but may not completely disappear.