Varicose veins may develop during pregnancy for a number of reasons. The additional volume of blood places additional pressure on the veins. As the uterus enlarges during pregnancy, it puts pressure on the inferior vena cava, the large vein on the right side of the body. This in turn increases pressure in the leg veins. Elevated levels of progesterone cause veins to relax, so that valves in the veins may not fully close and blood can flow back into the vein.