The foramen ovale (or ovalis) is the opening in the interatrial septum in the fetal heart that allows blood to bypass the right ventricle and non-ventilated lungs, shunted from the right atrium to the left atrium.
Specifically it represents the opening between the upper and lower portions of the septum secundum. The septum primum exists as a valve-like leaflet on the left atrial side of the foramen ovale, held open by the flow of blood from right atrium to left atrium. In the fetal circulation, the jet of oxygenated blood returning from the placenta via the ductus venosus is directed towards the foramen ovale, ensuring the fetal heart and brain get the most oxygenated blood 1.
In the immediate postnatal period, changes to the fetal circulation include 2:
- an increase in systemic vascular resistance due to the clamping of the umbilical cord, increasing the pressure in the left side of the heart
- a decrease in pulmonary vascular resistance due to vasodilation of the pulmonary capillary bed as the lungs are newly aerated, decreasing the pressure in the right side of the heart
- the loss of the jet of blood returning from the placenta via the ductus venosus
These changes result in the left atrial pressure becoming higher than the right atrial pressure, so the septum primum is pushed back against the foramen ovale to prevent a left-to-right shunt. This typically fuses by 3 months of age, forming a complete atrial septum.
After closure a small oval depression in the interatrial wall persists as the fossa ovale.
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