Dear Honeywell,
The risk of a blood clot crossing an atrial septal defect (ASD) will depend on the size of the ASD and the risk of clot formation in the individual. The risk is rather low in small ASDs and increases as the size increases. Things that you can do to reduce your risk of blood clot formation is to not smoke, not use birth control pills and avoid prolonged periods of sitting in one position. Your doctor may also advise you on taking aspirin on a regular basis. Hope this helps.