The foramen ovale is a small opening in the interatrial septum of the heart that allows blood to bypass the lungs in fetal circulation. During fetal development, the foramen ovale allows oxygenated blood from the placenta to enter the left atrium and then be pumped out to the body. After birth, when the lungs become functional, the foramen ovale usually closes, redirecting blood flow through the pulmonary circuit. However, in some individuals, the foramen ovale may remain partially open, causing a condition called a patent foramen ovale.