In our outpatient fetal ultrasound result consultation, abnormal fetal lung development is often found in mid-pregnancy. Fetal isolated lung is a more common problem and easy to cause concern for pregnant women and their families, so what is isolated lung and what will be the effect on the fetus? Isolated lung is a non-functional mass of lung tissue formed by the embryonic foregut, extra developed trachea and bronchopulmonary buds receiving blood supply from the body circulation, with no traffic with the normal airway, and its histological features are consistent with cystic adenoma-like malformation of the lung, which is a congenital malformation of the lung caused by embryonic developmental defects based on vascular anomalies, with a low incidence. Isolated lung is divided into extralobar and intralobar types, with intralobar isolated lung being rare in fetuses and mostly extralobar. Lobar isolated lung, also known as parietal lung, is separated from normal lung tissue and has its own pleural envelope with blood supply from the body circulation. 80% to 90% of isolated lung occurs at the base of the lung, at the bottom of the left thoracic cavity, with some occurring in the mediastinum, diaphragm, subdiaphragm or within the pericardium. Ultrasound is the most commonly used prenatal diagnostic method, and its ultrasound imaging may show: 1) a strongly echogenic or slightly echogenic mass in the fetal thorax or abdominal cavity, triangular or lobulated, with uniform internal echogenicity and clear borders; 2) color Doppler detection of its trophoblastic vessels from the arteries of the body circulation or its branches. Isolated lung needs to be distinguished from cystic adenomatous malformation of the lung, with the main difference being the origin of the trophoblastic vessels. The vast majority of fetal isolated lungs have a good prognosis, with approximately 40% of these lesions shrinking or disappearing spontaneously and a perinatal mortality rate of only 5%. Therefore, fetal isolated lung detected in prenatal diagnosis may not be a cause of great concern to the pregnant woman, and continuous dynamic follow-up is necessary rather than blind termination of pregnancy. The main risk factors affecting the prognosis are excessive size and growth of the mass, compression of the heart, lungs and inferior vena cava, leading to severe pulmonary dysplasia, heart failure and fetal edema, which threaten the life of the fetus, so the size and growth rate of the mass are more valuable than the pathological type. After prenatal ultrasound detects isolated lung, the growth rate of the mass and the presence of fetal edema should be closely observed to guide clinical treatment. Surgical treatment of symptomatic isolated lung after birth in the neonatal period is generally not controversial, but the decision to perform surgery for asymptomatic isolated lung can be made after birth depending on the situation.