Friends, prenatal fetal ultrasonography is a very important part of secondary prevention of birth defects. Because ultrasonography is non-invasive, convenient, inexpensive, real-time and dynamic, it has become the most commonly used imaging tool in clinical screening for fetal developmental malformations. Today, we will briefly discuss the role and time frame of fetal ultrasound during pregnancy to help mothers and fathers-to-be understand the importance of ultrasound and eliminate some misconceptions and confusion. I. Is fetal ultrasound examination safe? The principle of ultrasound is: sound waves pass through the uterus and meet the fetus and reflect back. The reflected echoes form an image on the display, thus allowing the ultrasonographer to see the position and movement of the fetus. When a pregnant woman has an ultrasound, the ultrasound equipment will emit a small amount of heat. The body of the pregnant woman, the amniotic fluid surrounding the baby and the baby’s own fetal movements all help to disperse the heat, and the heat generated by the ultrasound used for prenatal examinations is less than 1 degree Celsius, which is very safe for the baby. How many ultrasound examinations should be performed during pregnancy? At present, the number and timing of prenatal ultrasound examinations are recognized differently in different countries and regions. We suggest that under normal circumstances, pregnant women should have at least 5 ultrasound examinations during pregnancy: 1. early ultrasound screening within 8 to 11 weeks of pregnancy: the main purpose is to confirm the gestational week, determine intrauterine pregnancy or ectopic pregnancy, singleton or twin (to confirm chorionicity) or multiple fetuses, whether the embryo is alive, and whether there are abnormalities in the pregnant woman’s uterus and adnexa. 2.NT examination between 11 and 13 weeks + 6 days of pregnancy: NT is the thickness of the transparent layer of the fetal neck, which is the most important measurement in this pregnancy. 3.Fetal system ultrasound examination: This is the most important fetal ultrasound examination in the whole pregnancy, and its main purpose is: the screening of abnormalities of fetal system structures, including the screening of abnormal development of most fetal heart structures. Combining the recommendations of the International Society of Ultrasound in Obstetrics and Gynecology for the gestational week of fetal system screening and our work experience, we suggest that it should preferably be performed between 22 and 26 weeks of gestation. This is because the fetus is of moderate size during this period, the organs are basically developed, the morphological features of the face, heart, abdominal cavity and other organs are obvious, the bones have ultrasound reflexes, and the echogenic loss of the bones during this period has little effect, which is conducive to the display of fetal visceral structures, and the amniotic fluid is sufficient, the fetus is stretched, the mobility is large, the sonogram is clear, the malformations are relatively easy to be exposed, and the ultrasound is easier to observe the fetal systems. The fetal system is easier to be observed by ultrasound. 4.Fetal ultrasound examination around 32 weeks of pregnancy: it is mainly to monitor the intrauterine growth and development, amniotic fluid volume and fetal position. 5.Fetal ultrasound examination around 37 weeks of pregnancy: mainly to monitor intrauterine growth and development, amniotic fluid volume, estimate fetal weight, confirm fetal position and thus confirm the mode of delivery. Which hospitals should I go to for NT ultrasound examination? The thickness of the nuchal translucency (NT) is the most important indicator for screening chromosomal disorders during early pregnancy. The nuchal translucency refers to the subcutaneous fluid at the back of the fetal neck observed by ultrasound in early pregnancy, which is shown sonographically as an echogenic band between the skin and the subcutaneous soft tissue at the level of the fetal cervical spine, and is required to be performed between 11 and 13 weeks and 6 days of gestation and the fetal head-rump length is between 4.5 and 8.4 cm. Under normal circumstances, NT thickness increases with increasing fetal head-rump length, but its thickness should be less than 3.0 mm at 14 weeks. NT thickening is the most sensitive independent indicator for screening for Down syndrome in early pregnancy, and is also associated with other chromosomal aneuploidy, congenital heart disease and genetic syndromes. According to statistics, more than 90 fetal anomalous disorders are seen in NT thickened fetuses, including central nervous system, face, neck, heart, lung, abdominal wall, gastrointestinal tract, genitourinary, skeletal, neuromuscular malformations, fetal anemia, and metabolic defects. Since NT measurement requires a high degree of standardization in fetal position and ultrasound section, ultrasonographers without professional training (ultrasonographers need to be certified by the British Fetal Medicine Foundation) may result in excessive errors in the measurement values, which in turn increases the psychological burden on pregnant women and their families. Therefore, pregnant women should be sure to have their NT measurements performed at the appropriate week of pregnancy in a medical unit that is qualified to measure NT. Should we do 3D color ultrasound and 4D color ultrasound? When screening for fetal macrosomia, the detection rate of fetal structural abnormalities depends mainly on the level of the doctor, not on whether 3D or 4D technology is used. In fact, both the American College of Obstetricians and Gynecologists and the International Society of Ultrasound in Obstetrics and Gynecology routinely recommend 2D color ultrasound in their clinical guidelines for screening for macrosomia in mid-pregnancy. 3D ultrasound is only needed in special circumstances, such as when a more visual examination of fetal facial abnormalities is needed. 4-D ultrasound is a time axis added to 3-D imaging, which allows dynamic 3-D imaging to be seen in real time, and is only used in a few cases. Some medical institutions in China irresponsibly promote 3D or 4D color ultrasound as the most advanced high technology, using it as a signboard to solicit patients and recommending it as a routine to screen for large malformations, a practice that is very unscientific. V. Can fetal ultrasound screen out all malformations? Many pregnant women think that the fetal ultrasound examination during the whole pregnancy, especially the fetal system ultrasound examination, should be able to detect all fetal abnormalities or malformations, and medical disputes will arise in case of missed screening or missed diagnosis. We need to treat fetal ultrasound during pregnancy objectively and rationally. Regardless of the method used and the stage of pregnancy, it is unrealistic and unreasonable to expect all fetal abnormalities to be detected, even if the most renowned ultrasound specialists perform a thorough examination. In fact, in recent years, with the improvement in the sophistication and resolution of ultrasound diagnostic instruments, and with the improvement in the level of the obstetric ultrasonographer team itself, more and more fetal anomalies are being screened out, avoiding the birth of children with major birth defects. VI. What are the nagging ultrasound soft indicators? Some ultrasound soft indicators are closely related to fetal chromosomal abnormalities, such as thickened NT/NF, choroidal cyst, nasal bone hypoplasia, shortened long bones, bilateral lateral ventricle widening, pleural effusion, thickened liver echogenicity, echogenicity enhancement of both kidneys, bilateral renal pelvis separation, umbilical bulge, overlapping fingers, pinky second knuckle shortening, increased iliac pterygoid angle, enhanced intestinal echogenicity, single umbilical artery, focal strong echogenicity in the heart, and right subclavian artery vagalization have been shown to be associated with chromosomal abnormalities. However, there are other ultrasound soft indicators that we are more confused about, such as fetal choroidal cyst, intracardiac focal strong echo and right subclavian artery vagalization. Whether the presence of these indicators requires fetal chromosomal examination is still controversial. For example, fetal choroidal cysts often appear at 14 to 16 weeks of gestation and most disappear on their own by about 22 weeks of gestation; only rarely they persist into late pregnancy or even the neonatal period. It is generally accepted that fetal chromosome karyotyping should be performed when other abnormalities are detected by ultrasound in addition to choroidal cysts, whereas fetal chromosome karyotyping is not recommended if only a single choroidal cyst is detected. Due to the limitation of space, we cannot give examples of all the soft ultrasound indicators. Pregnant women should not be overly panic if they encounter these cases, but should choose a professional prenatal diagnosis institution for consultation and reasonable follow-up examination. Friends, it is the beautiful vision of every family to have a healthy baby, and please believe that all medical personnel who work with fetal ultrasound, including me, also carry such a beautiful expectation. I hope that mothers and fathers-to-be can correctly understand the role, timing, purpose and limitations of prenatal fetal ultrasound, and treat fetal ultrasound objectively and rationally. Let’s work together and trust each other to protect the health of our babies.