Prior to birth, oxygenated blood flows from the mother, through the placenta, to the heart of the foetus via the opening in the wall separating the left and right atrium (foramen ovale), into the foetal circulation. The foramen ovale has a “trap-door” feature which opens due to the pressure of blood flow from the mother’s placenta entering the right atrium and lets the blood pass to the left atrium. At birth, the lungs expand and the pressure in the left atrium increases and “slams shut” the foramen ovale. Shortly after birth the “door” fuses together, but it fails to fuse completely in roughly 27% of people and results in a patent foramen ovale, also called persistent foramen ovale (PFO).