A patent foramen ovale or PFO is a congenital heart defect consisting of a small hole between the right and upper left atria of the heart (the two upper chambers). It is normal in a newborn infant, and appears to be merely part of the normal growth of the heart in utero. However, once the newborn starts breathing, pressure in the left atrium starts to exceed that in the right atrium and tissue called the septum primum is forced against the hole. Eventually, the tissues fuse together, usually about three months after birth.