1, choroid plexus cyst (CPC): choroid is located in the lateral ventricle, the third ventricle, the fourth ventricle, is the site of cerebrospinal fluid production. CPC is a cyst that appears in the choroid plexus, most believe that the cause is the folds of the neuroepithelium in the choroid, containing cerebrospinal fluid and cellular debris, can be single or multiple, such as obstruction of cerebrospinal fluid circulation can cause ventricular dilatation. It has also been suggested that most cysts have a hemangioma-like capillary network and stroma, which are pseudocysts. CPC occurs in 1%-2% of normal fetuses and may appear transiently but disappears at 20 weeks. The sonogram is a round or oval anechoic structure seen within a homogeneous strongly echogenic choroidal plexus, mostly 3-5 mm in size, and the diagnosis should be considered for those found after 18 weeks with a diameter of 10 mm or more. The chance of chromosomal abnormalities in pure CPC is between 1 and 2.4%. Simple CPC disappears in late pregnancy and is in the vast majority not combined with other abnormalities. If combined with other abnormalities, especially multiple malformations, the chance of chromosomal abnormalities is high, including trisomy 18, trisomy 21, etc. Ventricular dilatation (ventriculomegaly): Cerebrospinal fluid is produced by the intraventricular choroid plexus and enters the third ventricle through the interventricular foramen, then flows into the fourth ventricle through the middle and lateral foramina, and then enters the subarachnoid space through the middle and lateral foramina. Various causes cause obstruction of cerebrospinal fluid circulation and accumulation in the ventricles, resulting in ventricular dilatation. A marked ventricular dilatation with a lateral ventricular width of ≥15 mm is called hydrocephalus. It is mostly due to narrowing of the midbrain aqueduct, and the causes include chromosomal abnormalities, inflammation, and mass compression. After 20 weeks of gestation, lateral ventricles or cerebellar medullary pools with a width of more than 10 mm should be alerted for ventricular dilatation hydrocephalus and should be followed up closely. When the width is >10 mm and <15 mm, it is called mild ventriculomegaly. The incidence is 1.5‰-22‰, mostly not due to obstruction of the ventricular system, and should be further examined in detail for extracranial lesions such as agenesis of the corpus callosum and cardiac malformations. Note that about 5-10% of fetuses with isolated mild ventricular dilatation are chromosomal abnormalities, among which trisomy 21 children are more common. 3, posterior cranial fossa pool widening (enlarged cisterna magna): also known as posterior cranial fossa pool widening, Magna bursa widening, refers to the distance between the fetal cerebellar pool and the anterior posterior diameter of the medial aspect of the skull ≥ 10 mm. Posterior cranial fossa pool widening is associated with fetal haploid abnormalities, especially trisomy 18, and is also seen in arachnoid cysts, Dandy-Walker malformation, etc. In the absence of other coexisting anomalies, ultrasound and other imaging studies are feasible for follow-up and observation. 4. pyelectasis/hydronephrosis: Urinary tract obstruction leads to urine retention in the renal pelvis and calyces, and ultrasound shows dilated anterior and posterior diameters of the renal pelvis. Severe pyelectasis can result in atrophy of the renal parenchyma and an increase in kidney size. Pelvic effusion has been reported to be detected in 2%-2.8% of normal fetuses and 17%-25% of children with trisomy 21. Anterior-posterior diameter of renal pelvis separation (anteroposterior diameter, APD) values of ≥4 mm at 15-20 weeks, ≥5 mm at 20-30 weeks, and ≥7 mm at 30-40 weeks may present as fetal anomalies and should be followed until after birth. Other organic pathologies include pelvic ureteral junction stenosis, ureteral bladder junction stenosis or ureteral dilatation due to vesicoureteral reflux, posterior urethral valves, and Prune-belly syndrome (urethral obstruction resulting in a large fetal bladder with extremely thin bladder wall and fetal abdominal wall).