Under normal circumstances, the secundum foramen closes after the child is born. Once the child is born, the left atrium pressure exceeds the right atrium, directly pressing the patch onto the foramen and closing it.
The fear lies in those people whose defect is too large, and the patch is not big enough to cover it, leaving it incompletely closed. This kind of person occupies a certain percentage of the population, and it's not low, reaching twenty to thirty percent. Another major category of atrial septal defect occurs here, called secundum atrial septal defect.
Upon hearing this, some people find it hard to believe that so many suffer from this disease because there's seldom news around them about such patients.
