I only know that in a prenatal development there is an oval foramen, which connects right and left atrium. I don't think that there is a passage between right atrium and left ventricle. Please correct me if Im wrong.

In the development period there is a connection between the two but as I'm aware of it's not functional during your development after birth

Thank you , but I think that there might be a chance that even after birth there is a really small amount of blood coming through septum atrioventriculare, but I really need to know if there is a chance that that is possible, even small amount is important

I think you may be referring to a patent foramen ovale? The foramen ovale is patent during foetal development and closes shortly after birth. The foramen ovale and ductus arteriosus allow blood to be shunted away from the pulmonary circulation as foetal blood is oxygenated by the mother. This closes when pulmonary vascular pressure rises at birth. The foramen ovale is a hole with 2 thin sheets of tissue either side. These are pushed together when the pressure increases and will then fuse. In some cases, the foramen ovale remains patent (not as rare as you may think, but is open to varying degrees and usually causes no significant symptoms). This can be diagnosed via echo or bubble echo. It is clinically significant as emboli from the venous circulation may enter the right atrium, transit to the left atrium via the PFO and enter the arterial circulation which may lead to stroke. edit: i just re-read and realised you’re talking about AVSD. There are varying degrees of AVSD, all of which require surgical intervention. Just remember that blood, like any liquid flows from high to low pressure. Depending on the precise nature of the defect the blood flow may be slightly different. Babies with AVSD will eventually be in cardiorespiratory distress. I think it’s alot more common in trisomy 21 patients but you might want to look that up.