It is a long process that every mother-to-be must go through before the birth of her baby. As an obstetrician who escorts the mothers-to-be, we understand their mixed feelings during this process, and every maternity checkup is a mixture of anticipation and apprehension. Ultrasound examination is an indispensable means to observe fetal structural abnormalities and developmental abnormalities during maternity checkups, and it also occupies an extremely important position in the minds of mothers-to-be. This is the most common inquiry after the examination. Recently, a patient was admitted to our department who was found to have fetal fluid in the abdominal cavity by ultrasound examination since 30 weeks of pregnancy. The newborn was diagnosed as congenital ileal atresia by the pediatric department and underwent ileal resection and end-to-end anastomosis of the atretic area. In this case, it occurred to me that many mothers-to-be are concerned about the ultrasound abnormality of fetal ascites, or fetal fluid in the abdomen. What are the causes? What is the prognosis of the fetus? This article is a brief introduction to the etiology and prognosis of fetal ascites in order to relieve some of these questions and confusion. Fetal ascites is an abnormal accumulation of fluid in the fetal abdominal cavity, and is one of the more common abnormalities in the fetus on ultrasonography. It can be divided into simple ascites and “fetal edema syndrome” with ascites. Simple ascites refers to the absence of fluid accumulation in other parts of the fetus except ascites, but it may be combined with malformation or other ultrasound abnormalities; whereas fetal edema syndrome refers to the abnormal accumulation of fluid in two or more different parts of the fetus, including pleural effusion, pericardial effusion, skin edema and placental thickening. The exact incidence is currently difficult to estimate because there are no uniform diagnostic criteria and mild or transient ascites may be missed. Because fetal ascites is often combined with organ or structural malformations involving multiple systems such as digestive, respiratory, skeletal, and cardiovascular, it is important to pay attention to the combination of abnormal sonograms of these systems when ultrasound suggests the presence of fetal ascites. The etiology of fetal ascites is complex, and it is generally accepted that both immunologic and non-immunologic causes of fetal edema syndrome may lead to the development of fetal ascites, in addition to idiopathic ascites of unknown origin. Immunologic causes are mainly maternal-fetal blood group incompatibility leading to fetal hemolytic anemia, which usually causes the typical syndrome of hydrops fetalis; non-immunologic causes include fetal chromosomal abnormalities, intrauterine infections, structural malformations of fetal organs and thalassemia. Other possible causes of fetal ascites include twin-to-twin transfusion syndrome (TTTS), maternal-fetal blood transfusion, maternal hyperthyroidism, special medications during pregnancy and some hereditary diseases. Among the diagnoses of the causes of fetal ascites made by ultrasound, the largest proportion of abnormalities of the digestive system, mainly fetal fecal peritonitis, and the rate of diagnosing the cause of fetal edema syndrome is higher than that of simple ascites. In addition, because cord blood provides more clues for etiologic diagnosis than amniotic fluid, such as ascites due to hematologic abnormalities, cord blood aspiration should be preferred for the etiologic diagnosis of fetal ascites. The prognosis of fetal ascites is related to many factors, and the type of fetal ascites (simple ascites or hydrops fetalis syndrome), the week of gestation at which the ascites appears, and whether it is combined with fetal anomalies are currently considered to be important factors affecting the prognosis of the fetus. It is generally accepted that fetuses with simple ascites have a good prognosis under prenatal monitoring until full term, as in the neonate in the aforementioned case. Fetuses with ascites presenting at less than 24 weeks of gestational age, with fetal edema syndrome or with other systemic anomalies tend to have a higher mortality rate. In addition, the etiology of fetal ascites is one of the most important factors in determining the prognosis of fetuses with ascites, and it is desirable to evaluate the prognosis of the fetus by etiologic diagnosis. For fetuses with ascites, laparotomy and reduction of ascites and intrauterine blood transfusion may be performed on a case-by-case basis, but their therapeutic value is unclear and their role in improving fetal prognosis is extremely limited. Through the above introduction, mothers-to-be should have a rough understanding of fetal ascites, and when fetal ascites is detected by ultrasound, they can be fully prepared psychologically and cooperate with the obstetrician to actively search for the cause while performing ultrasound examinations regularly, and make appropriate choices according to the gestational week, the cause, the presence of fetal malformations and the intrauterine condition of the fetus.