Mid-pregnancy ultrasound performed between 18 and 24 weeks of gestation reveals a number of well-defined fetal structural abnormalities and ultrasound soft indicators. Ultrasound soft indicators are nonspecific, often transient, and are seen in normal fetuses, but their incidence is elevated in fetuses with chromosomal abnormalities. Well-studied ultrasound soft indicators include choroid plexus cysts, intracardiac echogenicity, posterior cervical skin thickening, intestinal echogenicity, renal pelvic dilatation, short long bones, absent or hypoplastic nasal bones, mild lateral ventricular widening, and a single umbilical artery. Ultrasound soft indicators are not pathognomonic, but can be used to assess the risk of chromosomal abnormalities. I. Presentation and clinical significance of ultrasound soft indicators 1. Choroid plexus cysts: Choroid plexus cysts are seen in the axial plane of the fetal skull, located in the lateral ventricles, and may be single or multiple, unilateral or bilateral, and present as a restricted echogenic area within the choroid plexus. The presence of only 1 soft indicator, choroid plexus cyst, does not indicate an increased risk of chromosomal abnormalities. When ultrasonography reveals a combination of other structural abnormalities, the fetus is at increased risk for trisomy 18-trisomy, but not for trisomy 21-trisomy. Choroid plexus cysts are present in 1-2.5% of normal pregnancies. a large multicenter study by Chitty et al. randomized 1,600 pregnancies to 658 fetuses with choroid plexus cysts, with a risk of chromosomal abnormality (predominantly trisomy 18) that was 1.5 times that of the control group. a study by Comstock showed that 49,435 ultrasound examinations of pregnant women at 16-25 weeks’ gestation revealed fetal choroid plexus cysts in the fetuses. The Comstock study showed that 49,435 ultrasounds were performed at 16 to 25 weeks of gestation in 49,435 pregnancies, and 1,209 fetal choroid plexus cysts were detected (2.3%), of which 1,060 had choroid plexus cysts alone without other structural anomalies; 50 fetuses with trisomy 18 were ultimately diagnosed, and half of them had choroid plexus cysts. The study concluded that the detection of choroid plexus cysts requires careful examination of other fetal structures, especially the hands, to detect the presence of overlapping fingers and clenched fists to aid in the exclusion of trisomy 18. If the fetus is not associated with other structural abnormalities, the risk of trisomy 18-trisomy will not be increased; if it is accompanied by other structural abnormalities, chromosomal karyotyping by amniocentesis is recommended.2. Intracardiac strong echoes: Intracardiac strong echoes refer to tiny foci of calcification with echoes similar to the intensity of bone that are seen in the papillary muscles or any of the ventricles, which can be present in a single ventricle or in two ventricles, and may be singular or multilocular. On ultrasound, it should be noted that multiple angles must be visualized to determine intracardiac echoes, except for specular echoes in the papillary muscles.Intracardiac echoes are seen in 1.5% to 4% of fetuses. Pseudointracardiac echoes are often seen in the regulatory bundle, endocardial cushion, and tricuspid annulus. In order to correctly identify strong intracardiac echoes, the following methods are recommended: (1) they are present within the ventricles of the papillary muscles; (2) they are visible from multiple planar angles; (3) they are independent of the papillary muscle specular reflex zone; and (4) they should not manifest as an inlet-outlet reflex.Coco et al. summarized ultrasonographic findings from 12,672 pregnant women in mid-pregnancy and concluded that strong intracardiac echoes in the fetus do not increase the risk of chromosomal abnormalities.Filly et al. showed that of 21,839 low-risk pregnancies for trisomy 21, 626 (2.9%) had only intracardiac strong echoes, and only 1 of these fetuses had trisomy 21.Rochon and Eddleman showed that in the low-risk group for chromosomal abnormalities, only one indicator, intracardiac strong echoes, was not associated with trisomy 21, and that even if there was a risk, the risk was much lower than the risk of fetal loss associated with interventional prenatal diagnosis. Even if there is a risk, it is much lower than the risk of fetal loss associated with interventional prenatal diagnosis. Therefore, the detection of strong intracardiac echoes by ultrasound in pregnant women younger than 35 years of age is a normal physiologic manifestation, and amniocentesis is not recommended. After finding typical intracardiac strong echoes, meticulous ultrasound examination of the fetus is needed to clarify whether there are structural abnormalities, and if combined with other obvious structural abnormalities or soft indicators, amniocentesis is recommended.3. Posterior nuchal skin thickening: ultrasound detection of thickening of the fetal nuchal skin in the 15th to 23rd week of gestation is one of the earliest ultrasound soft indicators found in the middle stage of gestation, and it is one of the most predictive value indicators as well. Early studies suggested that a posterior nuchal skin thickness of ≥6 mm was indicative of a risk of fetal chromosomal abnormalities. Another study utilized a statistical analysis of subjects’ work characteristic curves and concluded that a cut-off value of >5 mm of posterior cervical skin thickness was recommended before 20 weeks of gestation. Recent studies have found that the value of postnuchal skin thickness increases with gestational week, and specific cut-off values need to be established for different gestational weeks.Smith-Bindman et al. showed that thickening of the postnuchal skin increased the risk of trisomy 21, with a likelihood ratio of 17 (95% CI: 8-38). The incidence of posterior nuchal skin thickening is lower if the fetal nuchal translucency thickness is normal in early pregnancy. In addition, posterior nuchal skin thickening may also be an early manifestation of fetal hydrops or lymphoedema.4. Strong echogenicity of the intestinal canal: In 1990, Nyberg et al. and Persutte firstly reported strong echogenicity of the intestinal canal in the fetus. In mid-pregnancy, ultrasonography found that fetal intestinal echoes were consistent with the echoes of the adjacent bone before the diagnosis of strong intestinal echoes. Fetal echogenicity can be categorized as focal, multifocal or diffuse echogenicity. When examining fetal bowel echoes, the probe frequency should not be higher than 5 MHz.Once suspected bowel echoes are detected, the ultrasound gain should be gradually reduced until only the bone and bowel are visible. A grading method of strong echoes of the bowel has been proposed to minimize the differences between different examiners.Slotnick and Abuhamad compared the intensity of strong echoes of the bowel with that of the iliac crest and categorized them into 3 grades: grade 1 refers to the intensity of the bowel echoes being lower than that of the iliac crest; grade 2 refers to the intensity of the bowel echoes being the same as that of the iliac crest; and grade 3 refers to the intensity of the bowel echoes being higher than that of the iliac crest.The association of strong echoes of the bowel with chromosomal aneuploidy and poor pregnancy outcome is more significant in grades 2 and 3 Grade 2 and 3 echoes are more closely associated with chromosomal aneuploidy and adverse pregnancy outcomes. The incidence of mid-gestational echogenicity ranges from 0.2% to 1.4%. It may occur in normal fetuses, chromosomally abnormal fetuses, fetal growth restriction, early gestational cf, cystic fibrosis, congenital viral infections, or thalassemia.Bromley et al. found that mid-gestation intestinal echoes were seen in only 0.6% of fetuses; however, intestinal echoes were present in approximately 15% of trisomy 21 fetuses.Sepulveda and Sebire found that intestinal echoes in fetuses with strong echoes were more commonly associated with chromosomal aneuploidies and adverse pregnancy outcomes. Sepulveda and Sebire found that pathological changes were present in about 35% of fetuses with strong echogenicity of the bowel. Bleeding in early pregnancy may also result in echogenicity due to ingestion of blood by the fetus. If echogenicity is detected, a careful examination of the fetus is required. Amniocentesis is recommended to determine the karyotype and the presence of cytomegalovirus, toxoplasmosis, and microvirus infections, and the mother should be examined for recent cytomegalovirus and toxoplasmosis infections. Dynamic ultrasound monitoring is recommended because of the possibility of concurrent fetal growth restriction.5. Renal pelvis dilatation: Fetal renal pelvis dilatation is more common in the middle of pregnancy, with an incidence of 0.3% to 4.5% (average of about 1%). Mild renal pelvic dilatation means that the renal pelvis width is between 4 and 10 mm and there is no dilatation of the renal calyces. Fetuses with renal pelvis widths ≥10 mm or hydronephrosis are at risk for structural anomalies and require continued evaluation.In 1990, Benacerraf et al. were the first to find that renal pelvic dilatation was associated with chromosomal abnormalities, with mild renal pelvic dilatation occurring in 25% of trisomy 21 fetuses and in 2.8% of normal fetuses.In a prospective multicenter study by Chudleigh et al., 101,600 Chudleigh et al. showed that 101,600 pregnant women underwent ultrasonography and 737 fetuses were found to have mild renal pelvic dilatation, of which 12 (1.6%, 12/737) had chromosomal abnormalities (9 combined with other ultrasonographic abnormalities, 1 pregnant woman with advanced maternal age, and 2 with mild renal pelvic dilatation only). In a retrospective study of 25,586 pregnancies by Havutcu et al, 320 fetuses (1.3%) had renal pelvic dilatation without chromosomal abnormalities, 19 had other ultrasound abnormalities, and 301 had renal pelvic dilatation only. Other studies have also demonstrated that the presence of only one soft indicator, renal pelvic dilatation, did not correlate significantly with fetal chromosomal abnormalities. This suggests that fetal renal pelvis dilatation should not be used as an indication for amniocentesis in the absence of other structural abnormalities or risk factors. However, fetal renal pelvic dilatation progressively worsens in approximately 1/4 to 1/3 of fetuses, increasing the risk of hydronephrosis and neonatal urinary reflux; therefore, ultrasonography is recommended to determine fetal renal pelvic dilatation in late pregnancy, and postnatal evaluation or surveillance is indicated if it persists or worsens. Fetal renal pelvis widths of 4 to 7 mm found in mid-gestation generally do not require surgical intervention.6. Short long bones: Fetal short long bones can be used as an indicator of chromosomal abnormalities, and fetuses with short femurs and humeri are at risk for developing Body 21. Short femur index measurement/expected value ≤ 0.91, short humerus index measurement/expected value ≤ 0.89. Studies have shown that in fetuses with trisomy 21, 24%~45% have short femur and 24%N54% have short humerus; while in normal fetuses, only 5% have short long bones. It is found that short humerus has more predictive value than short femur, and only short humerus is more meaningful than short humerus and short femur at the same time, therefore, measurement of humerus length should become a routine item of mid-pregnancy ultrasonography.7. Nasal bone defects or dysplasia: mid-pregnancy ultrasonography can check the nasal bone in the mid-sagittal plane of the fetal head. Nasal bone hypoplasia refers to the length of the nasal bone <2.5 mm. Bromley et al. found that the incidence of nasal bone defects in normal fetuses and fetuses with trisomy 21 was 0.5% and 43%, respectively, and the likelihood ratio of the nasal bone defects in predicting the risk of trisomy 21 was 83, which is the soft indicator with the highest sensitivity. Sonek et al. found that the incidence of nasal bone defects in normal fetuses and fetuses with trisomy 21 in mid-gestation was 1% and 37%, respectively, with a positive likelihood ratio of 41 and a negative likelihood ratio of 0.64. Therefore, it is considered that nasal bone defects are very important ultrasound indicators with important predictive value for trisomy 21.8 Mild Lateral Ventricular Widening: The width of the lateral ventricles is normally 10 mm or less, and when it ranges between 10 and 15 mm, it is defined as mild lateral ventricular widening. The incidence of mild lateral ventricular widening is 0.15% in chromosomally normal fetuses and 1.4% in trisomy 21 fetuses, with a likelihood ratio of 9. Lateral ventricular widening increases the risk of chromosomal abnormalities in the fetus and increases the likelihood of neurological abnormalities in the long-term development by 10% to 30%. Chang Ching-hsien et al. showed that the prognosis was better in fetuses with lateral ventricular dilatation width of 10.0-12.0 mm. If fetal lateral ventricular dilatation is found, the fetal structure should be carefully examined and amniocentesis is recommended, as well as screening for fetal infection indicators, and fetal neurological MRI should be performed if necessary to find out whether there is a combination of other intracranial developmental anomalies, such as hypoplasia of the corpus callosum or obstruction of the ventricular system, etc. 9. Single umbilical artery: Single umbilical artery refers to the presence of one umbilical artery and one umbilical vein in the umbilical cord. The current view is that single umbilical artery does not increase the risk of chromosomal abnormality if the fetus is not associated with other structural abnormalities. However, dynamic observation is necessary to be vigilant for the occurrence of fetal cardiac and renal developmental abnormalities and fetal growth restriction. Shen Lin and Wu Lianfang showed that single umbilical artery is an important indicator of poor fetal outcome if accompanied by severe fetal growth restriction. Second, prenatal counseling for ultrasound soft indicators Some studies have estimated the change in likelihood ratio by using an information assessment system , which suggests that the more types of ultrasound soft indicators appear, the greater the risk of fetal chromosomal abnormalities, and the higher the likelihood ratio value, but the method has not been effectively applied in the clinic. Currently, in the group with low risk of serologic screening for trisomy 21, if there is a single ultrasound soft indicator or multiple ultrasound soft indicators, the concept of likelihood ratio can be introduced, and based on the value of the likelihood ratio, combined with the results of the serologic screening for a comprehensive assessment, if there is a risk of chromosomal anomalies, it is recommended that interventional prenatal diagnosis be performed. It should be noted that the majority of ultrasound soft markers have a likelihood ratio for chromosomal abnormalities only for trisomy 21, whereas some ultrasound soft markers may be associated with other chromosomal abnormalities, e.g., choroid plexus cysts are associated with trisomy 18, and this should be clearly explained to the pregnant woman and her family. Ultrasound soft indicators can help determine whether further fetal chromosome testing is needed. The presence of 2 or more ultrasound soft indicators requires attention and evaluation, and consideration of interventional prenatal diagnosis, except for chromosomal abnormalities.