The foramen ovale is a small hole located in the atrial septum that is used during fetal circulation to speed up the travel of blood through the heart. When in the womb, a baby does not use its own lungs for oxygen-rich blood; it relies on the mother to provide oxygen rich blood from the placenta through the umbilical cord to the fetus. Therefore, blood can travel from the veins to the right side of the baby's heart and cross to the left side of the heart through the foramen ovale and skip the trip to the baby's lungs.
Normally the foramen ovale closes at birth when increased blood pressure on the left side of the heart forces the opening to close.
If the atrial septum does not close properly, it is called a patent foramen ovale. This type of defect generally works like a flap valve, only opening during certain conditions when there is more pressure inside the chest. If the pressure is great enough, blood may travel from the right atrium to the left atrium. If there is a clot or particles in the blood traveling in the right side of the heart, it can cross the PFO, enter the left atrium, and travel out of the heart and to the brain causing a stroke, or into a coronary artery causing a heart attack.