A surgeon creates an arteriovenous fistula by making a connection between an artery (which carries blood away from the heart) and a vein (which carries blood back to the heart). This artificial connection allows the vein to become larger and for the walls of the vein to thicken, a process termed maturation. A mature fistula makes it easier for the vein to be punctured repeatedly for dialysis. Maturation typically takes three to six months to occur, but in rare cases, can take up to a year. This makes advance planning for an arteriovenous fistula important. When a patient is felt to be approximately a year away from requiring dialysis, the patient should be referred for evaluation for possible creation of an arteriovenous fistula.