Foramen ovales are holes in the heart needed before birth to transfer oxygenated blood from the umbilical cord to the unborn child, bypassing the lungs, which are not used by the fetus. Soon after birth, this hole generally closes, but in about 20% of people, it does not close completely. This remaining opening is called a patent foramen ovale (PFO).

Many PFOs are relatively small and do not cause significant problems, but in some people, the opening is larger and can provide a pathway for blood clots to more easily reach the brain and cause a stroke. In these cases, treatment may be necessary to correct the defect.