Atrial septal defect is a relatively common congenital heart disease in infants and toddlers. It is indeed possible for a child's atrial septal defect to close on its own. Some atrial septal defects can close as the child grows. Depending on the severity, atrial septal defects can be categorized into small defects. Generally, if the defect is less than 5 millimeters, it is possible for the defect to close as the child develops. If it has not closed by the age of two, it generally will not close on its own thereafter.