The NT refers to the maximum thickness of the soft tissue between the skin layer and the fascial layer on the back of the fetal neck, which reflects the accumulation of lymphatic fluid in the subcutaneous tissue. The NT is the maximum thickness of the soft tissue between the skin layer of the back of the neck and the fascial layer. NT examination is better at 11-13+6 weeks. The normal thickness of NT increases with the number of weeks of gestation and is generally considered to be abnormal at 11-13+6 weeks of gestation ≥2.5 mm; and at 14-22 weeks ≥6 mm. It may be relaxed in advanced pregnancies. The causes of NT widening may be related to genetics, anatomical abnormalities or infection leading to lymphatic reflux disorders, and some may develop into cervical lymphatic hydrocystic tumors by mid-pregnancy. The most common chromosomal abnormality in early NT widening is trisomy 21, in addition to trisomy 18, trisomy 13, trisomy 22, 45XO (Turner’s syndrome), etc. In addition, non-chromosomal abnormalities such as fetal heart malformations, fetal edema, thoracic dominant lesions, skeletal dysplasia, and recipient of twin-birth transfusion syndrome should be excluded. In general, about 80%-90% of NT abnormalities are transient lesions and the fetus is normal. 2, choroid plexus cyst (choroid?plexus?cyst, CPC) can be detected by ultrasound after 10 weeks of gestation, and it is believed that the cause is the folds of neuroepithelium in the choroid, containing cerebrospinal fluid and cellular debris. It may be solitary or multiple, and may cause ventricular dilatation if it obstructs the cerebrospinal fluid circulation. CPC occurs in 1%-2% of normal fetuses and may appear transiently, mostly disappearing at 20 weeks. There is no clear pathological significance and the prognosis is good. those found after 18 weeks with a diameter of 10 mm or more should be considered for diagnosis. The number of chromosomal abnormalities in simple CPC is 1 to 2.4%. Simple CPC will disappear in late pregnancy and the vast majority are 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. Cerebrospinal fluid is produced by the intraventricular choroid plexus and enters the third ventricle through the interventricular foramen, then the fourth ventricle through the middle cerebral aqueduct, and then 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 to hydrocephalus and should be followed closely. If the width is >10 mm and <15 mm, it is called mild ventricular dilatation (mild?ventriculomegaly). It is mostly due to non-ventricular systemic obstruction, increasing the possibility of malformation, and TORCH examination and karyotype examination are recommended. About 5-10% of fetuses with isolated mild ventricular dilatation are chromosomally abnormal, with trisomy 21 children being more common. 4, posterior cranial fossa pool widening (enlarged? cisterna? magna) The echogenic area between the posterior part of cerebellum and cerebellar earthworm and the inner surface of occipital bone is the posterior cranial fossa pool, and the posterior cranial fossa pool ≥10mm is considered as posterior cranial fossa pool widening. Widening of the posterior cranial fossa pool is associated with fetal haploinsufficiency, especially 18-trisomy, and is also seen in arachnoid cysts and Dandy-Walker malformations. In the absence of other coexisting abnormalities, ultrasound and other imaging studies are feasible for follow-up. 5. Mild pyelectasis (pyelectasis) Mild pyelectasis refers to an increase in the anterior and posterior diameter of the separated renal pelvis but is not sufficient to diagnose hydronephrosis. Severe pyelectasis can cause atrophy of the renal parenchyma and increase in kidney size. Anterior-posterior diameter of renal pelvis separation (anteroposterior?diameter, APD) values of ≥4 mm at 20 weeks, ≥5 mm at 20-30 weeks, and ≥7 mm after 30 weeks are considered mild pelvic dilatation with possible fetal anomalies and should be followed until after birth. Other organic lesions include pelvic ureteral junction stenosis, ureteral bladder junction stenosis or ureteral dilatation due to vesicoureteral reflux, posterior urethral valves, Prune-belly syndrome (urethral obstruction resulting in a huge fetal bladder with extremely thin bladder wall and fetal abdominal wall), etc. 6. single umbilical artery (SUA) The normal umbilical cord contains two umbilical arteries and one umbilical vein. sua refers to only one umbilical artery and the incidence is about 1%. The larger one is the umbilical vein and the smaller one is the umbilical artery, which is slightly larger than the normal lumen. SUA can occur singly or in combination with chromosomal abnormalities and other malformations, such as trisomy 18 and trisomy 13. SUA is associated with a significantly increased risk of cardiac malformations, renal malformations and FGR. Further fetal echocardiography is recommended for clinical findings of SUA. 7. Echogenic intracardiac focus (EIF) EIF is an isolated focal echogenic spot on a four-chamber cardiac image in the free area of one ventricular cavity, corresponding to the papillary muscle or tendon area, with an echogenic intensity similar to that of the fetal skeleton. It may be solitary or multiple, with the left ventricle being the most common, gradually decreasing with gestational age, and disappearing by the end of the first year. The cause may be related to inflammation, thickening or calcification of the papillary tendon cords, which generally does not affect health or cardiac function, is a normal variant and is common in Asians. The incidence of chromosomal abnormalities in fetuses with EIF is about 1/600 when the maternal age is ≥31 years. 8. Short femur length is considered as one of the characteristics of chromosomal abnormalities, and femur is the only long bone routinely measured by obstetric ultrasound scan. The femur is the only long bone routinely measured on obstetric ultrasound. A BPD/FL greater than 1.5 is used as a criterion to detect 54-70% of children with trisomy 21. Short femur in middle and late pregnancy is also seen in chondrodysplasia, FGR, infants younger than gestational age, and congenital proximal femur defects. 9. Hyperechogenic bowel is not a disease but a sonographic manifestation of enhanced fetal intestinal echogenicity with intensity close to or higher than that of bone echogenicity, commonly seen in the small intestine of mid- and late-pregnancy fetuses. The incidence in mid- and late-term pregnancies is 1%. Most fetal follow-up results are ultimately normal, but a significant proportion of fetuses are confirmed to have abnormalities, such as chromosomal abnormalities, gastrointestinal abnormalities, intestinal obstruction, meconium peritonitis, cystic fibrosis, intra-amniotic hemorrhage, and intrauterine infection. 10. Increased or decreased fetal eye spacing Excessive or small orbital spacing in both eyes is commonly associated with some anomalous syndrome. A rough estimate of the central orbital distance (mm) is approximately equal to the number of weeks of gestation and can be judged accordingly. The fetal canthus index = (inner canthus/outer canthus) × 100, when the canthus index is ≥38, the canthus index is too large, which can be seen in 13-trisomy, 18-trisomy, 21-trisomy; when the canthus index is <20, the canthus index is too small, which can be seen in forebrain anomaly (whole forebrain), juxtacranial anomaly, microcephaly, and often also in 13-trisomy, 21-trisomy. The fetal nasal bone begins to develop in the 6th week of embryonic life and ossifies by membranous osteogenesis in the 9th-11th week, 1.4% of normal fetuses have missing nasal bone. In approximately 50-60% of trisomy 21 children, nasal bone defects are detected 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 ocular hyperfusion in fetuses with juxtacranial anomalies may manifest as single nostril, elephantine nose and abnormal nasal position. The standard measurement plane is the median sagittal plane in the horizontal position of the fetus, which shows a thin line of strong echogenicity in the nasal bridge. It is important to note that nasal bone defects can occur in a small percentage of chromosomally normal fetuses, and the background prevalence of nasal bone defects in chromosomally normal populations depends on the ethnicity and facial features of the parents. The distance between the corners of the fetal mouth increases or decreases in relation to gestational age. The increased distance between the corners of the mouth is 2 standard deviations higher than normal and is seen in a variety of chromosomal abnormalities. The decrease in the distance between the corners of the mouth, which is 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 developmental malformations Absence of mandible or small mandibular malformation is often one of the frequent abnormalities of chromosomal anomalies syndrome. The anterior-posterior and left-right diameters of the fetal mandible are reduced and significantly lower than those of normal fetuses of the same gestational age. Ultrasonography can clearly show the horseshoe-shaped mandible. In the past, visual inspection of the facial contour was more subjective, but now the jaw index is more commonly used to determine this. The mandibular index (jaw?index) = (anterior-posterior mandibular diameter/biparietal diameter) × 100. mandibular index <21 in small mandibular deformities, commonly seen in trisomy 18, trisomy 21, 45XO, 5P deletion, etc. Chromosomal abnormalities have been reported in about 66% of small mandibular fetuses, and autopsy reports the presence of small jaw anomalies in 80% of triploid children. 14, amniotic slice Ultrasonography during pregnancy reveals a strongly echogenic band of light floating in the amniotic fluid within the amniotic sac, called amniotic slice or uterine cavity adhesion folds. This is due to the presence of adhesion scars in the uterine cavity and the growth of amniotic and chorionic membranes along the stretched scars. It is thicker because it contains two layers of chorionic villus and two layers of amnion, so the echogenicity is strong and obvious. It is associated with the increasing number of uterine operations in women of childbearing age today. The amniotic slice is not associated with fetal adhesions and is not associated with fetal malformations and requires no special treatment. However, care should be taken to differentiate it from other intrauterine band echoes, such as amniotic band syndrome, incomplete longitudinal uterine septum, contoured placenta, amniotic sac separation in multiple fetuses, etc. The causes of fetal anomalies are diverse, and there is no effective prevention method in medical science, but only through early diagnosis and timely termination of pregnancy. Ultrasonography is the first choice for early diagnosis of fetal anomalies. Some of these microscopic anomalies appear early and persist, some are transient, some appear irregularly, and some are late onset lesions. Although many fetuses with chromosomal abnormalities do not show any signs on ultrasound images. However, as soft ultrasound markers of chromosomal abnormalities, they can provide clues for careful screening of fetal anomalies and alert the operator to carefully examine the fetus for combined abnormalities elsewhere. For continued pregnancy, the sonographic changes should be followed regularly. Although the probability of problems occurring in the isolated presence of the above-mentioned phenotypes is small and the sensitivity and specificity are not high; however, for older women with abnormal serologic screening results (PAPP-A, α-FP, β-hCG, uE3, inhibin-A) and combined with other high-risk factors, chorionic villus biopsy (10-13 weeks), amniocentesis (16-22 weeks), in addition to magnetic resonance examination, should eventually be performed (16-22 weeks), cordocentesis, and other interventional methods to extract fetal cells for karyotype analysis, in order to clarify the diagnosis.