Atrial septal tumor is not a tumor! It is simply a structural abnormality of the atrial septum protruding into one side of the atrium. Its incidence has been reported in the literature to be about 0.2% to 1.1%, and it is less commonly found in adults and children in clinical practice. In recent years, with the rise of fetal cardiac ultrasound, more “diagnoses” have been made in the fetal period, but most of them are not clinically significant. In the fetus, the fossa ovalis valve (secondary to the atrial septum) is located on the left atrial side and is offset toward the left atrium to open the foramen ovale, constituting a right-to-left blood flow channel. It is possible that due to the small size of the foramen ovale, the excessive length of the oval valve and the weak development of the oval valve, the oval fossa valve projects toward the left atrium forming a so-called atrial septal distention tumor, which is a short-term process during partial fetal development. The prognosis is generally good and will disappear with the closure of the foramen ovale after birth, and should not be induced as a malformation. Very few of them can be maintained into adulthood and may be combined with atrial defect. In our hospital, 112 cases of fetal septal expansion tumor were followed up, and the longest follow-up was one year, all of them were healthy and growing. Health care measures: 1. No special treatment or care is needed during the fetal period; 2. Normal delivery is possible without special delivery measures; 3. Cardiac ultrasound is performed after birth. Treatment: Most of them do not require treatment. Long-term prognosis: Fully normal life, study and work.