There are two general types of atrioventricular septal defect (AVSD) that can occur, depending on which structures are not formed correctly: Complete AVSD A complete AVSD occurs when there is a large hole in the center of the heart which allows blood to flow between all four chambers of the heart. This hole occurs where the septa (walls) separating the two top chambers (atria) and two bottom chambers (ventricles) normally meet. There is also one common atrioventricular valve in the center of the heart instead of two separate valves – the tricuspid valve on the right side of the heart and the mitral valve on the left side of the heart. This common valve often has leaflets (flaps) that may not be formed correctly or do not close tightly. A complete AVSD arises during pregnancy when the common valve fails to separate into the two distinct valves (tricuspid and mitral valves) and when the septa (walls) that split the upper and lower chambers of the heart do not grow all the way to me...