Fetal Echocardiography Practice Test

ASD located in the inferior part of the interatrial septum is the ostium primum defect. This opening sits in the lower section of the septum primum, the tissue that grows down from the atrial roof toward the endocardial cushions. During normal development, the ostium primum closes as the septum primum fuses with the endocardial cushions; if closure fails, a persistent opening remains in that inferior area, creating an ostium primum ASD. The term septum primum refers to the embryologic structure itself, not the defect type, so naming the defect as septum primum isn’t correct. Ostium secundum defects, by contrast, occur higher up near the fossa ovalis and relate to the septum secundum.