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 method for screening fetal developmental malformations due to its advantages of being non-invasive, convenient, inexpensive, real-time, dynamic, etc. At present, the initial systematic ultrasound at 18-22 weeks of gestation is used to detect congenital malformations. At present, the majority of fetal morphological and structural abnormalities can be detected by initial systematic ultrasound screening at 18-22 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, intraventricular strong echogenic spot, short femur, strong echogenicity of the intestinal tubules, abnormalities of the nasal bones, and micrognathia. 1, nuchal translucency thickness (NT): NT refers to the maximum thickness of the soft tissue between the dorsal skin layer and the fascial layer of the fetal nuchal, 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. 14 weeks later, the lymphatic system is well developed, and the accumulated lymphatic fluid is rapidly drained to the internal jugular vein, and NT disappears. 16 weeks later, it is renamed as the nuchal skin fold thickness (nuchal skin fold thick). The NT should be performed at 11-14 weeks. The acoustic image shows an echogenic layer under the skin of the neck. The commonly used criteria are: ≥2.5mm at 11-14 weeks of gestation is considered abnormal; ≥6mm at 14-22 weeks of gestation is considered abnormal. Appropriate relaxation can be made for older pregnant women. Lymphatic reflux disorders due to genetics, anatomical abnormalities, or infections are the causes of NT widening, and in some cases, it may develop into cervical lymphocystic hygroma in the mid-pregnancy. Chromosomal abnormalities have been reported in 10% of cases of early NT widening, mainly trisomy 21, trisomy 18, trisomy 13, 45X0 (Turner’s syndrome), etc. In addition, cardiac abnormalities should be excluded. Non-chromosomal abnormalities such as cardiac malformations, hydrops fetalis, thoracic space-occupying lesions, skeletal dysplasia, and twin-transfusion syndrome recipients should also be excluded. In general, about 80%-90% of NT abnormalities are transient lesions and the fetus is normal. 2, choroid plexus cyst (CPC): choroid is located in the lateral ventricle, the third ventricle, the fourth ventricle, is the place to produce cerebrospinal fluid, CPC is the cysts appearing in the choroid plexus, most of them are thought to be caused by the folds of neuroepithelium in the choroid, which contains the cerebrospinal fluid and cellular debris, and they can be single or multiple, such as blockage of the circulation of the cerebrospinal fluid can result in the dilatation of the cerebral ventricles. It has also been suggested that most cysts have angiomatous capillary networks and stroma in the wall, which are pseudocysts. The incidence of CPC is 1-2%, and it may appear transiently in normal fetuses, but mostly 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 found after 18 weeks. The chance of chromosomal abnormality 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 anomalies, especially multiple malformations, the chance of chromosomal abnormality is very 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 the fetuses with isolated mild ventricular dilatation were chromosomal abnormalities, in which trisomy 21 was common. 4. Widening of the posterior cranial fossa (enlarged cisterna magna): also known as widening of the posterior cranial fossa, widening of the Magna bursa, refers to the distance between the cerebellar fossa and the anterior-posterior diameter of the medial side of the cranium of the fetal cerebellum is ≥10mm. Widening of the 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 abnormality, ultrasonography 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 is only one, the incidence of about 1%, the left side of the missing than the right side of the common. 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 a point-like isolated focal echo on the four-chambered image of the heart in the free area of one ventricular cavity, equivalent 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 is recommended. Short femur length: short long bone dysplasias is considered to be one of the characteristics of chromosomal abnormalities, and the femur is the only long bone that is routinely measured by obstetric ultrasound scanning. If the femur is smaller than the fifth percentile for the corresponding gestational week and other growth indicators are normal, this is a high priority. 19% of trisomy 21 babies have a short femur. A BPD/FL greater than 1.5 is used as a criterion to detect 54%-70% of trisomy 21 infants. Short femur in middle and late pregnancy is also seen in chondrodysplasia, IUGR, less than gestational age, and congenital proximal femoral defect (PFFD). 9, intestinal strong echo (hyperechogenic bowel): not a disease but a sonographic manifestation, refers to the fetal intestinal echogenic enhancement, its intensity is close to or higher than the bone echogenicity similar to the small intestine in mid-pregnancy fetus and late pregnancy fetus colon. The incidence in mid- and late-term pregnancies is 1%. Most fetuses have ultimately normal follow-up results, but a significant proportion of fetuses have confirmed abnormalities, such as chromosomal abnormalities, digestive tract malformations, intestinal obstruction, meconium peritonitis, cystic fibrosis, intra-amniotic hemorrhage, intrauterine infections, and so on. 10.Fetal eye spacing increases or decreases: the orbital distance between the eyes is too large or too small, which is commonly seen in some kind of anomalous syndromes. Roughly estimated orbital center distance (mm) is about equal to the number of gestational weeks, and can be judged accordingly. Fetal canthus index = (inner canthus/outer canthus) × 100, when the canthus index ≥ 38, for the canthus index is too large, can be seen in trisomy 13, trisomy 18, trisomy 21; when the canthus index < 20, for the canthus index is too small, can be seen in the forebrain anencephaly (whole forebrain holoprosencephaly), and eye deformity, small head deformity, and is often also the performance of trisomy 13, trisomy 21. 11. Fetal hypoplasia (nasal hypoplasia) and absence of nasal bone: the fetal nasal bone starts to develop in the 6th week of embryonic period, and ossifies through membranous osteogenesis in the 9th-11th weeks, and 1.4% of normal fetuses have absence of nasal bone. Approximately 50-60% of trisomy 21 infants will have a missing nasal bone on ultrasound screening at 10-14 weeks. Fetal nasal widening or narrowing can also be seen in a variety of chromosomal abnormalities. For example, anterior anencephaly and juxtacanthalactic fetal ocular hyperfusion may be manifested by single nostrils, elephantine nose, and abnormal nasal position. The standard plane of measurement is the median sagittal plane in the horizontal position of the fetus, which shows a thin line of strong echoes within the nasal bridge. Normal values of nasal bone length have been reported variably, generally ranging from 1.3-2.1 mm for CRLs of 45-84 mm. It is important to note that nasal bone deletions occur in a small proportion of chromosomally normal fetuses, and the background incidence of nasal bone deletions in chromosomally normal populations depends on parental ethnicity and facial features. 12. Increased or decreased distance between the corners of the mouth: The distance between the two corners of the mouth of the fetus correlates with gestational age. When the distance between the corners of the mouth is increased by 2 standard deviations above normal, a large mouth deformity is present, which can be seen in a variety of chromosomal malformations. For example, chromosome 2 long arm duplication, chromosome 9 short arm duplication; while the mouth angle distance decreases, lower than 65% of normal fetuses of the same gestational age, is also often a clinical manifestation of chromosomal and genetic syndromes. 13, fetal mandibular development malformation: no jaw (agnathia) or small jaw malformation (micrognathia/micrognathism /small jawz/small chin) is often a chromosomal abnormality syndrome of one of the most common anomalies. In micrognathia, the anterior-posterior, left-right, and left-right diameters of the mandible are reduced and are significantly lower than those of a normal fetus of the same gestational age. The horseshoe-shaped mandible can be clearly visualized by ultrasonography. In the past, the method of visual inspection of the facial contour was more subjective, but now the jaw index is used to judge. Jaw index = (anteroposterior mandibular diameter/biparietal diameter) × 100. Small mandibular deformities with jaw index <21 are commonly seen in trisomy 18, trisomy 21, 45XO, and 5P deletion. It has been reported that about 66% of fetuses with small mandibles have chromosomal abnormalities, and autopsy reports that 80% of triploid children have small jaw malformations. 14, amniotic film: ultrasound examination during pregnancy found that the amniotic sac has a strong echo band floating in the amniotic fluid, called amniotic film also known as the uterine cavity adhesion folds (uterine shelf). This is the result of an adhesion scar in the uterine cavity, and the amnion and chorionic villi grow along the stretched scar. It is thick and contains two layers of chorionic villi and two layers of amniotic membrane, and is therefore strongly echogenic and distinct. The incidence has been reported to be 0.6% in the literature, although it is not uncommon in recent years and should be related to the increasing number of uterine operations in women of childbearing age. Amniotic membrane sheet is not adherent to the fetus, and also has nothing to do with fetal malformation, no special treatment. However, care should be taken to differentiate it from other intrauterine banded echoes, such as amniotic band syndrome, incomplete longitudinal septum of the uterus, contoured placenta, and amniotic sac septum in multiple fetuses. The causes of fetal malformations are varied, and currently there is no effective prevention method, but only through early diagnosis and timely termination of pregnancy after discovery. Ultrasonography is the first choice for early diagnosis of fetal malformations. Some of the above-mentioned microscopic malformations are early and persistent, some are transient, some occur irregularly, and some are late-onset. Although many fetuses with chromosomal abnormalities do not show any signs on ultrasound imaging. However, as ultrasound soft-markers for chromosomal abnormalities, they can provide clues for careful screening for fetal malformations, alerting the operator to double-check the fetus for other abnormalities. In continuing pregnancies, sonographic changes are reviewed periodically. Although the probability of problems occurring in the isolated presence of the above appearances is small, and the sensitivity and specificity are low; however, in those older pregnancies with abnormal serologic screening (PAPP-A, α-FP, β-hCG, uE3, and inhibin-A) results and a combination of other high-risk factors, in addition to magnetic resonance testing, ultimately chorionic villus biopsy (10-14 weeks), amniocentesis Chorionic villus biopsy (10-14 weeks), amniocentesis (16-24 weeks), umbilical cordocentesis and other interventional methods should be performed to extract fetal cells and then perform chromosomal karyotyping to make a definitive diagnosis.