The wall between the upper chambers of the heart, the atria, consists of two overlapping layers of tissues, the septum primum and septum secundum. When the two layers fail to normally fuse together shortly after birth, the flap opening is called a PFO which is a Latin derived medical term – Patent (to lie open) Foramen (window) Ovale (oval in shape). Before birth everyone has a PFO that allows blood to bypass the baby’s lungs because the mother, via the placenta, provides oxygen to the baby’s blood. At birth the flap shuts and fuses shut in three out of four individuals similarly I would say 4/5 close.