Ostium primum ASD's are rarer than the ostium secundum defects but are often more serious. The ASD is low in the atrial septum, just above the atrioventricular valves.
Commonly the anterior leaflet of the mitral valve is bifid, resulting in incompetence.
More serious defects involve the atrial septum, both atrioventricular valves and the ventricular septum. This is called an atrioventricular canal defect.
Serious ASD's of this type are found with other congenital abnormalities such as Downs syndrome, asplenia and polysplenia.