**6. Embryology**

In the past, failure of the fusion of endocardial cushions was thought to be the only reason for AV septal defects [16]. It could be the first step in the formation of these hearts, but not in entirety. Delamination of valve leaflets occurs late in development, with its formation occurring by undermining of the ventricular myocardium [17].

When the endocardial cushions fail to meet, subaortic outflow tract will not be normally wedged and there will be abnormal development of ventricular mass. Additionally, mesenchymal tissues surrounding the primum ostial foramen play a role in these defects [18, 19].
