Congenital heart defects are structural problems with the heart present at birth. They result when a mishap occurs during heart development soon after conception and often before the mother is aware that she is pregnant. They can involve the interior walls of the heart, the valves inside the heart, and/or the arteries and veins that carry blood to the heart or the body. Defects range in severity from simple problems, such as “holes” between chambers of the heart, to very severe malformations, such as complete absence of one or more chambers or valves.