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In the fetal circulation, a physiologically normal hole in the heart wall between the right atrium and the left atrium.

It allows oxygenated, nourished blood to flow from the umbilical vein through the inferior vena cava through the right atrium to the left atrium to be pumped into the systemic circulation without going through the pulmonary circulation.

This structure normally closes at birth. This occurs as the pressure in the two atria change as the lungs expand as the baby takes its first breaths, causing the flap to snap shut. Failure of the foramen ovale to close results in a patent foramen ovale.