Fetal Development
The first major system to form and function during fetal development is the cardiovascular system. The heart begins to beat on the 23rd day of gestation and by the 49th day the heart is completely formed.
The fetus receives oxygen via the placenta before it is born; the foramen ovale is a hole between the left and right atrium that allows blood to bypass the lungs prior to birth. This foramen (or hole) closes once a baby is born and if it does not close it is called a patent foramen ovale (PFO).