In fetal life, 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, which constitutes a normal right-to-left blood flow channel. Generally, the size of the foramen ovale is 4-6 mm; less than 4 mm means that the foramen ovale is small. The mechanism of occurrence is unknown and may be related to the development of the foramen ovale. Since the foramen ovale is the blood flow channel for the left atrium and left ventricle during fetal life, once the foramen ovale becomes too small, the development of the left heart, especially the development of the left ventricle, should be evaluated regularly and cautiously, and once the development of the left ventricle is stalled, the pregnancy should be terminated as soon as possible. The fetus encountered with this condition generally has a good prognosis and should not be induced for malformation. In our hospital, we followed up 18 cases of small fetal foramen ovale, reviewed fetal heart ultrasound every two weeks, and delivered by cesarean section at the right time when the left ventricle development became slow or stagnant around 36 weeks of gestation, and the fetus showed normal left ventricular development by ultrasound one week after birth. Health care measures: 1. Fetal heart ultrasound is repeated every two weeks to closely observe the development of the left ventricle; 2. Normal delivery can be awaited and cesarean delivery is chosen when appropriate; 3. Heart ultrasound is performed after birth. Treatment: Generally no treatment is needed. A few children with small left ventricular development may be considered for supportive treatment such as moderate dobutamine. Long-term prognosis: Fully normal life, study and work.