Every year, 800,000 to 1.2 million children are born in China with congenital malformations, accounting for 4% to 6% of the total births in China, which has become a serious social and public health problem. Prenatal ultrasound has become the most commonly used imaging tool for clinical screening of fetal developmental malformations due to its advantages of being non-invasive, convenient, inexpensive, real-time and dynamic. At present, the majority of fetal morphological and structural abnormalities can be detected by initial systematic ultrasound screening at 18-24 weeks of gestation. However, the abnormalities detected by ultrasound include micro-malformations (also known as potential chromosomal markers) in addition to the obvious severe malformations. A correlation between these microscopic anomalies and fetal chromosomal abnormalities (mostly aneuploid aneuploid) has been reported in the literature. Common microscopic anomalies include increased nuchal translucency, choroidal plexus cysts, ventricular dilatation, widening of the renal pelvis, single umbilical artery, intracardiac echogenic spot, short femur, strong echogenicity of the intestines, abnormalities of the nasal bone, and micrognathia. 1.Nuchal translucency thickness (NT): NT refers to the maximum thickness of soft tissue between the skin layer and the fascia layer at the back of the fetal nuchal region, reflecting the accumulation of lymphatic fluid in the subcutaneous tissue. Before 14 weeks of gestation, the fetal lymphatic system is not well developed, and a small portion of lymphatic fluid collects in the cervical lymphatic sacs or lymphatic vessels, forming NT.NT examination should be performed at 11-14 weeks. The acoustic image shows a subcutaneous echogenic layer in the neck. The commonly used criteria are: ≥3mm at 11-14 weeks of gestation is considered abnormal; after 14 weeks, the lymphatic system is well developed, and the accumulated lymphatic fluid is rapidly drained to the internal jugular vein, and NT disappears; after 16 weeks, it is renamed as nuchalskin fold thick (NF), and ≥6mm at 16-22 weeks is considered abnormal. Impaired lymphatic return due to genetics, anatomical abnormalities, or infection is the cause of NT widening, and in some cases, it may develop into cervical lymphocystic hygroma (CCH). Chromosomal abnormalities have been reported in 10% of early NT widening cases, mainly trisomy 21, trisomy 18, trisomy 13, and 45XO (Turner’s syndrome). In addition, cardiac anomalies, hydrops fetalis, thoracic space-occupying lesions, skeletal dysplasia, twin transfusion syndrome of the recipient child and other non-chromosomal anomalies should be excluded. Choroid plexus cyst (CPC): choroid plexus is located in the lateral ventricle, third ventricle and fourth ventricle, which is the place of cerebrospinal fluid (CSF) production; CPC is cysts appearing in the choroid plexus, which is thought to be caused by the folds of neuroepithelium in the choroid plexus, which contains CSF and cellular debris, and can be singular or multiple, and can result in dilatation of the brain ventricles if it obstructs the circulation of the cerebrospinal fluid. It has also been suggested that most of the cysts are pseudocysts with angiomatous capillary network and stroma in the wall.The incidence of CPC is 1-2%, and it may appear transiently in normal fetuses but disappears at 20 weeks. The sonogram shows a round or oval anechoic structure within a homogeneous, strongly echogenic choroidal plexus, mostly 3-5 mm in size, and the diagnosis should be considered in cases of 10 mm or more in diameter detected after 18 weeks of gestation. The chance of chromosomal abnormalities in simple CPC is 1 – 2.4%. Simple CPC disappears in late pregnancy, and most of them are not associated with other abnormalities. If combined with other abnormalities, especially multiple malformations, the chance of chromosomal abnormality is high, including trisomy 18 and trisomy 21. Ventriculomegaly: Cerebrospinal fluid is produced by the choroid plexus within the ventricles of the brain and enters the third ventricle through the interventricular foramen, then flows into the fourth ventricle through the midbrain aqueduct, and then enters the subarachnoid space through the middle foramen and lateral foramen. Ventricular dilatation occurs when cerebrospinal fluid circulation is obstructed for various reasons and accumulates in the ventricles. Significant ventricular dilatation with a lateral ventricle width of ≥15 mm is called hydrocephalus. Hydrocephalus is most often caused by narrowing of the midbrain aqueduct, and the causes include chromosomal abnormalities, inflammation, and mass compression. After 20 weeks of gestation, a lateral ventricle or cerebellar medullary pool that is more than 10 mm in width should be alerted to ventricular dilatation with hydrocephalus, and should be followed up closely. A width of >10 mm and <15 mm is called mild ventriculomegaly. The incidence is between 1.5 per thousand and 22 per thousand, which is mostly not caused by obstruction of the ventricular system, and should be further examined in detail for intracranial and extracranial lesions, such as agenesis of the corpus callosum, cardiac malformations, and so on. Note that about 5%-10% of fetuses with isolated mild ventricular dilatation are chromosomal abnormalities, in which trisomy 21 is common. Enlarged posterior cranial fossa (enlarged cisterna magna): also known as enlarged posterior cranial fossa, enlarged Magna bursa, refers to the distance between the cerebellar fossa and the anterior and posterior diameters of the medial side of the cranium of the fetus and the cerebellar fossa is ≥10 mm. enlarged posterior cranial fossa is associated with fetal haploid anomalies, especially trisomy 18, and is also seen in arachnoid cysts, Dandy-Walker anomalies and so on. If there is no other coexisting anomalies, ultrasound and other imaging examinations are feasible for follow-up observation. 5. Pyelectasis/hydronephrosis: Urinary tract obstruction leads to urine retention in the renal pelvis and calyces, and ultrasound shows that the anterior and posterior diameters of the renal pelvis are dilated. Severe pyelectasis can result in atrophy of the renal parenchyma and increased kidney size. Renal pelvic effusion has been reported to be detected in 2%-2.8% of normal fetuses and 17%-25% of trisomy 21 infants. 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 show fetal anomalies and should be followed up until after birth. Other organic lesions include pyeloureteral junction stenosis, ureteral dilatation due to uretero-vesical junction stenosis or vesicoureteral reflux, posterior urethral valves, and Prune-belly syndrome (urethral obstruction resulting in a huge fetal bladder with extremely thin bladder wall and fetal abdominal wall). 6. single umbilical artery (SUA): the normal umbilical cord contains two umbilical arteries and one umbilical vein. SUA refers to the umbilical artery with only one umbilical artery, the incidence is about 1%, and the absence of the left side is more common than that of the right side. Sonographically, only two lumens are seen in the umbilical cord, the larger being the umbilical vein and the smaller being the umbilical artery, which is slightly larger than the normal lumen. The umbilical arteries are slightly larger than the normal lumen. Color Doppler can also be used to show the umbilical arteries originating from the iliac arteries on either side of the bladder at the root of the umbilical cord. SUA can occur singly, but combinations of chromosomal abnormalities and other malformations are not uncommon, and SUA is present in approximately 50% of trisomy 18 infants and in 10-50% of trisomy 13 infants. a significant increase in risk for cardiac malformations, renal malformations, and IUGR has been recently reported in SUA. Further fetal echocardiography is recommended. 7. Intraventricular echogenic spot or echogenic intracardiac focus (EIF): EIF is an isolated focal echo on the four-chamber image of the heart in the free area of one ventricular cavity corresponding to the papillary muscles or tendon cords, with an echo intensity similar to that of the fetal skeleton (ribs). It may be single or multiple, and is most common in the left ventricle, diminishing with gestation and disappearing by one year of age at the latest. It may be associated with inflammation, thickening, and calcification of the papillary tendon cords, but is not inherently detrimental to health or cardiac function. It is a normal variant and is common in Asians. The incidence of EIF on ultrasound at 18-22 weeks of gestation is 2%-5%, 16%-30% in trisomy 21 infants, and 39% in trisomy 13 infants.The risk of EIF is increased when accompanied by other ultrasound abnormalities, but the chance of fetal abnormality is low in those presenting alone; the incidence of chromosomal abnormality in a fetus with EIF is about 1/600 in a pregnant woman ≥31 years of age. echocardiography. Summary: Although the probability of problems occurring in the isolated presence of the above appearances is small, and the sensitivity and specificity are not high; however, in pregnant women who are older and have abnormal serologic screening (PAPP-A, α-FP, β-hCG, uE3, inhibin-A) results in combination with other risk factors, chorionic villus biopsy should eventually be performed (10-14 weeks) in addition to magnetic resonance imaging, Chorionic villus biopsy (10-14 weeks), amniocentesis (16-24 weeks), umbilical cordocentesis (after 24 weeks), and other interventional methods of fetal cell extraction followed by chromosomal karyotyping should be performed to make a definitive diagnosis.