Final answer:
Twin-to-Twin Transfusion Syndrome (TTTS) occurs when there is an arteriovenous shunt within the placenta of monochorionic twins, and should not be confused with the usual fetal circulatory shunts like the foramen ovale and ductus arteriosus which close after birth.
Step-by-step explanation:
The statement 'Twin twin transfusion exists when there is an arteriovenous shunt within the placenta' describes a condition specific to monochorionic twins, where the twins share a single placenta with interconnected blood vessels. This condition is known as Twin-to-Twin Transfusion Syndrome (TTTS), not to be confused with the normal circulatory shunts found in the fetal circulatory system, which include the foramen ovale, ductus arteriosus, and ductus venosus. These shunts redirect blood flow to bypass certain organs like the lungs and liver of the fetus, which are not yet functioning as they would after birth. The arterial and venous shunting in TTTS, on the other hand, leads to an imbalance where one twin receives too much blood flow, and the other too little. While these fetal shunts close shortly after birth, the placenta, being a vital organ during pregnancy, supplies oxygenated blood through the umbilical vein and aids in the removal of waste via the umbilical arteries, all while preventing the mixing of maternal and fetal blood.