It is the defect in inter-atrial septum that is situated between 2 upper chambers of heart (right and left atriums). It is usually there in newborns in the form of PFO and PFO is essential for fetal survival. Majority of PFO’s close in some time.

Varieties of ASD 
1. Ostium Secundum (OS-ASD)
2. Ostium Primum
3. Sinus venosus (SVC and IVC types) defect
4. Coronary sinus type defect