The ostium secundum atrial septal defect accounts for 7% of all congenital heart lesions. Persistence of the ostium secundum is the most common atrial septal defect. This method avoids the symptoms that accompany most ostium secundum atrial septal defects. As the septum primum grows, the ostium primum progressively narrows. An ostium secundum ASD accompanied by an acquired mitral valve stenosis is called Lutembacher's syndrome. The ostium secundum progressively enlarges and the size of the septum primum diminishes. The ostium secundum provides communication between the atria after the ostium primum closes completely. This allows access to the sphenoid ostium and sphenoid sinus. As the septum primum grows, the ostium primum narrows and eventually closes. The septum primum eventually fuses with the endocardial cushion, closing the ostium primum off completely. An ostium (plural ostia) in anatomy is a small opening or orifice. About 10 to 20% of individuals with ostium secundum ASDs also have mitral valve prolapse. Some embryologists postulate that the ostium secundum may be formed through programmed cell death. The opening to a porocyte is a pore known as an ostium.