Veins return blood from your body to your heart. To return blood to your heart, the veins in your legs must work against gravity. Muscle contractions in your lower legs act as pumps, and elastic vein walls help blood return to your heart. Tiny valves in your veins open as blood flows toward your heart then close to stop blood from flowing backward. As you get older, your veins can lose elasticity causing them to stretch. The valves in your veins may become weak, allowing blood that should be moving toward your heart to flow backward. Blood pools in your veins and your veins enlarge and become varicose. Pregnancy can also be a cause of varicose veins.