Placental cyst is considered as a benign secondary tumor of the placenta, clinically about 10%-20% of placenta combined with cysts, often between 0.5-2cm in diameter, occasionally as large as 8-10cm. Placental cysts can be divided into two categories according to their origin and location: (1) amniotic cyst, located on the fetal side of the placenta under the amniotic membrane and chorionic vasculature, often formed by adhesions of the amniotic folds, mostly solitary, sometimes with a diameter of several millimeters to several centimeters, the cysts contain yellow transparent fluid and can be agglutinated and formed close to the umbilical cord. (1) amniotic cyst, located on the fetal side of the placenta under the amniotic membrane and chorionic vasculature, may be close to the umbilical cord attachment, often formed by the adhesion of the amniotic folds, mostly solitary, ranging from several millimeters to several centimeters in diameter, the cyst contains yellow transparent fluid, sometimes coagulation and bleeding, mostly does not affect the function of the placenta. (2) Chorionic cyst, often located at the chorionic end of the maternal lobe septum, so also known as septal cyst, is a common small cyst in the placental tissue, mostly seen in edematous placenta, diabetes mellitus or placenta with maternal-fetal Rh blood type incompatibility, may be formed by liquefied cystic degeneration after ischemia at the tip of the placental lobe septum. The cysts are round or ovoid in shape, with a diameter of several millimeters to 1 cm, and the fluid is mostly colorless, transparent or yellowish, occasionally bloody, and generally has no adverse effect on placental function. Most of the placental cysts are small and protrude towards the amniotic cavity, so there is no obvious pressure on the placenta and the placental function is not affected; no special treatment is needed. Placental cysts are not a contraindication to vaginal delivery, but larger cysts are at risk of rupture and bleeding during delivery, so the indications for cesarean section can be relaxed appropriately. At the same time, we should pay attention to the rupture of placental cysts during pregnancy, because it can cause localized placental bleeding and even placental abruption in severe cases, which can endanger the safety of mother and child, so close observation is needed. The clinical diagnosis of placental cysts mainly relies on ultrasound, which allows real-time dynamic observation of the source, size, internal echogenicity and blood flow around the cyst. The sonographic features are: cysts are often single, ranging from a few millimeters to several centimeters in diameter; clear borders, envelope echogenicity, and a homogeneous echogenic zone within the cyst, with good translucency and no color Doppler signal in the cyst wall and base; poor translucency in the fluid area of the cyst if combined with cyst wall hemorrhage; and protrusion of the placental-fetal surface cyst into the amniotic cavity. Placental cysts should be distinguished from the following placental masses: 1. placental venous blood pool It is the erosion and dissolution of the adjacent meconium tissue by the syncytial trophoblast cells of the placental villi, forming the villi gap, the small uterine spiral artery opening in the villi gap, and at the same time the villi gap has small uterine venous reflux. The ultrasound image shows a large sub-circular liquid dark area in the placental parenchyma with blood flow signal, and the blood flow spectrum can be detected by Doppler ultrasound. 2, placental hemangioma ultrasound can be seen in the placenta fetal surface round mass, with envelope, clear boundary, hypoechoic, uneven echogenicity, reticular or striated echogenicity, blood flow signal can be detected. 3, fetal sacrococcygeal spinal bulge Located at the interruption of two parallel light bands of the fetal spine, where the cystic mass expands dorsally, without blood flow, mostly accompanied by excessive amniotic fluid. 4. Umbilical cord cyst A part of the umbilical cord is inflated, with cystic anechoic, thin, smooth and neat cyst wall, cyst protruding outward, ultrasound shows no blood flow signal inside the cyst, and normal umbilical vessels can be seen next to it. Umbilical cord cysts near the root of the umbilicus should be distinguished from umbilical ureteral cysts in the umbilical cord, which are connected to the bladder and contain fetal urine, and the diagnosis can be confirmed based on urine leakage from the umbilicus of the newborn after the umbilical cord is broken at birth. Ultrasound identification is difficult and further examination after birth is required to confirm the diagnosis.