That one way valve essentially closes, but in some cases, it does not actually close. And what that means is we can still have a movement of blood between the two atrium of the heart. So, once again, during fetal development, there is a valve like structure that exists between the right and the left atrium. And this allows the oxygen rich blood to bypass those nonfunctional lungs within that developing fetus. Now, normally at birth, this valve shuts close. But in some fetuses, the foramen ovaly fails to close properly, which leads to a permanent valve system that connects the two atrium of the heart.