Babies with atrial septal defects do not necessarily develop more slowly. An atrial septal defect is one of the congenital heart diseases. If the diameter of the defect is relatively small, it will not affect the normal blood flow of the heart, the oxygen supply to the body, or significantly impact daily activities like sleeping or exercising for the baby. The growth and development of the baby would be normal, with no abnormalities compared to their peers. However, if the diameter of the atrial septal defect is relatively large, typically exceeding 6-8 millimeters, it can cause the baby to experience a lack of oxygen during physical activities, leading to a compromised quality of life and potentially causing delayed development.