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The closure of the foramen ovale is primarily initiated by an increase in pressure within the left atrium after birth. When a newborn takes its first breaths, the lungs expand and the pulmonary circulation increases, resulting in decreased resistance in the pulmonary arteries. This change allows more blood to flow into the left side of the heart, raising the pressure in the left atrium. As the pressure in the left atrium exceeds that in the right atrium, it pushes the flap of tissue that makes up the foramen ovale closed, effectively sealing it and preventing blood from bypassing the lungs.

This physiological process is essential for the normal transition of circulation from fetal to postnatal life. It allows the blood to flow to the lungs for oxygenation instead of bypassing them, as was necessary during fetal development. Other factors, although related to the overall physiological changes after birth, do not directly cause the closure in the same manner as the increased left atrial pressure does.