How often do adults get diagnosed with an atrial septal defect that wasn't found as a child?

It happens: Generally there are 2 reasons for the delay in diagnosis: it's very small, doesn't need treatment, and doesn't matter or the individual has not been under the type of medical care that would suggest it, ie where chest xrays, ekgs, or echos are done. In 30 years, i've just found a few.

Not sure: About 1 in 4 people have a pfo, a patent foramen ovale, or small opening between the atria. In most people, this is not significant, but in some it can result in tias - transient ischemic attacks - or even strokes and must be treated medically or surgically.