“What causes atrial septal defect?”

During Development the walls the atrium grow in stages, part of this is to allow oxygenated blood from mom to bypass the lungs of the fetus which cannot work until delivery and the baby inhales air. There are different patterns of failure of the system to grow and then close correctly. Small defects or holes take longer to cause symptoms and thus adults usually have smaller defects.

An ASD is congenital. It results from the failure of a fetal connection to close. When in utero, you are a fish. As soon as you are born, your fish-like circulation closes and new connections are formed to make you an air-breathing organism. Most ASDs are small and require no treatment, but you should discuss with your doctor.