Atrial septal defect is a common congenital heart malformation in clinical practice, accounting for about 5-10% of congenital heart disease, and can occur alone or in combination with other types of cardiovascular malformations. The disease is predominantly found in female patients, and the early symptoms are mild and not easily detected, so many patients will miss the time for treatment as a result. So, the following is to introduce how the atrial septal defect is formed, I hope it will help you to prevent the disease. First, developmental defects Primary foramen ovale type atrial septal defect is caused by defects in the development of the endocardial cushions during embryonic development, forming a large half-moon shaped atrial septal defect bit, in the anterior lower edge of the coronary sinus defect adjacent to the mitral annulus often accompanied by mitral valve cleft. Second, body circulation factors Secondary foramen ovale septal defect is mostly found among adult patients. Due to the pressure step difference between the normal left and right atria, oxygenated blood from the left atrium is shunted to the right atrium through the defect, and the blood flow in the body circulation is reduced, thus the child’s development is delayed and physical activity is somewhat limited. After the oxygenated blood enters into the pulmonary circulation, it can cause intimal hyperplasia of small pulmonary vessels and mid-layer hypertrophy and other lesions, resulting in the increase of pulmonary artery pressure and pulmonary vascular resistance, but the process is slow. Genetic factors About 15% of atrial septal defect cases are related to genetics, especially chromosomal translocation and aberrant trisomy syndrome. Most congenital heart disease is formed by the interaction of multiple genes and environmental factors. Environmental factors Intrauterine infections, radiation exposure, metabolic disorders, hypoxia, drugs, etc. can cause atrial septal defect. If a pregnant woman suffers from a viral or bacterial infection in the first trimester, especially rubella virus or coxsackie virus, the likelihood of the baby being born with congenital heart disease will increase. V. Other factors Some congenital heart disease is more frequent in highland areas, and some have significant differences in incidence between the sexes, indicating that the altitude and sex of the place of birth are also related to the occurrence of the disease. Understanding the causes of atrial septal defect plays a very important role in the prevention of the disease, and experts remind us that if we want our children to stay away from atrial septal defect, parents should prevent it as early as possible. Pregnant women and their spouses need to stop smoking, alcohol and other bad habits, in addition to actively doing prenatal checkups, preventing colds, avoiding drugs that have been shown to have teratogenic effects and being exposed to toxic and harmful substances.