In large and medium-sized cities in China, women are basically able to have an ultrasound examination during pregnancy. In some larger cities and more economically developed coastal areas, prenatal ultrasonography has become a routine examination during pregnancy, and routine ultrasonography can be performed at least once during pregnancy. However, standardized obstetric ultrasonography is still far from being popular in China, and in some economically underdeveloped remote areas, standardized obstetric ultrasonography is still far from being popularized.In 2012, the Ultrasonographers Branch of the Chinese Medical Doctors’ Association published the Guidelines for Prenatal Ultrasonography, which divides obstetric ultrasonography into early pregnancy ultrasonography [including ordinary ultrasonography during early pregnancy, ultrasonography of the nuchal translucency layer (NT) thickness of the fetus at 11-13+6 weeks layer (NT) thickness ultrasonography], middle and late pregnancy ultrasonography (including level I, II, III, IV obstetric ultrasonography), limited obstetric ultrasonography, consultation or expert level obstetric ultrasonography, the content and focus of obstetric ultrasonography are different for each pregnancy and each level [2]. I. Early pregnancy ultrasonography 1.1 Early pregnancy general ultrasonography 1.1.1 Indications Confirmation of intrauterine pregnancy, clinical suspicion of ectopic pregnancy, clarification of gestational week, diagnosis of multiple pregnancies, understanding of embryonic or fetal conditions (survival or death), early pregnancy bleeding to investigate the cause of early pregnancy, early pregnancy lower abdominal pain to investigate the cause of early pregnancy, assessment of maternal pelvic masses, uterine abnormalities, clinical suspicion of hyperemesis gravidarum, and assisted chorionic villus biopsy. 1.1.2 Contents of examination 1.1.2.1 Gestational sac Requires observation of the location, number, size and morphology of the gestational sac. Comprehensively scan the uterus and double adnexa area to know the location and number of gestational sacs and minimize the missed diagnosis of multiple pregnancies, uterine horn pregnancies and ectopic pregnancies. The internal diameter of the gestational sac (excluding the strong echogenic ring) is measured in the largest longitudinal and transverse sections of the gestational sac. The sum of the maximum anterior-posterior, left-right, and upper-outer diameters is divided by 3. The average internal diameter of the gestational sac grows at a rate of about 1 mm/d at 5 to 7 gestational weeks, and in the case of multiple pregnancies, chorionicity and amnionicity need to be clarified. If the average internal diameter of the gestational sac is >25mm by transabdominal ultrasound or >20mm by transvaginal ultrasound, and no yolk sac or embryonic echoes are seen in the sac, embryonic abruption should be considered. Transabdominal ultrasound examination of the average internal diameter of the gestational sac ≤ 25mm or transvaginal ultrasound examination of the average internal diameter of the gestational sac ≤ 20mm, the sac did not see the yolk sac and embryonic echoes, need to be repeated ultrasound review in 1~2 weeks. Intrauterine gestational sac should be differentiated from uterine fluid: there is no obvious double ring sign in uterine fluid, and the surrounding strong echoes are the separated endometrium; if there is uterine fluid and no gestational sac in the uterus, it is necessary to be vigilant against the occurrence of ectopic pregnancy, and detailed examination of bilateral adnexa should be carried out. Human chorionic gonadotropin (HCG) is positive, and there is no gestational sac echo in the uterus, there can be three situations: too small gestational week, ectopic pregnancy or miscarriage. Ectopic situation should be examined in detail, and vaginal ultrasonography needs to be recommended for those with high suspicion of ectopic pregnancy. 1.1.2.2 Measurement of head and hip length and observation of fetal heartbeat A series of transverse and longitudinal sections should be performed to conduct a comprehensive scan of the gestational sac to observe the number of embryos or fetuses; the head and hip length should be measured in the section of the embryo’s maximal long-axis or in the mid-sagittal section, when the fetus is in a natural extended position, without hyperextension or hyperextension. the head and hip length of the embryo in the 5th-7th gestational weeks grows at the rate of about 1mm/d. the length of the embryo in the vaginal ultrasound ≤ 5mm or in the abdominal ultrasound ≤ 5mm. If the embryo is ≤5mm long by transvaginal ultrasound or ≤9mm long by transabdominal ultrasound and the fetal heartbeat cannot be observed, the embryo needs to be followed up and reexamined after 7-10d. If the embryo length is >5mm by transvaginal ultrasound or >9mm by transabdominal ultrasound and the fetal heartbeat cannot be observed, embryonic arrest should be considered. The fetal heartbeat is <100 beats/min until 6+3 weeks of gestation, then it gradually accelerates to 180 beats/min at 9 weeks of gestation, and then slows down to about 140 beats/min at 14 weeks of gestation. 1.1.2.3 Uterus and adnexa Observation of uterine morphology, echoes of the myometrium, and the presence of uterine cavity effusion; and the presence of masses in both adnexa. If there is any mass, measure the size of the mass and observe the shape, border, cysticity, blood supply, relationship with ovary and uterus, etc., and evaluate the nature of the mass. 1.2 NT ultrasound at 11-13+6 weeks By the 1990s, Nicolaides developed a method of screening for trisomy 21 using measurement of the nuchal translucency.Michailidis et al.[3] found that 93.7% of fetuses had complete anatomy that could be visualized on early pregnancy ultrasound using two-dimensional ultrasound.Jones[4] concluded that more than 80% of fetuses had complete anatomies on early pregnancy ultrasound. concluded that more than 80% of fetal malformations are manifested by the 12th gestational week.The 2011 article "The New Pyramid Model of Obstetric Examination - Based on the 11-13 Weeks Examination" by Nicolaides published in Prenat Diagn states that scientific advances in the last two decades have made it possible to predict many pregnancy complications as early as 12 weeks of gestation. can be predicted as early as 12 weeks of gestation, which will change the pattern of labor and delivery, and the new pattern of labor and delivery is the inverted pyramid pattern, where high-density labor and delivery examinations will be scheduled at 11 to 13+6 weeks [5]. This will mean that prenatal checkups at 11 to 13+6 weeks will be the inevitable trend. prenatal ultrasound at 11 to 13+6 weeks is one of the most important of these many prenatal checkups. For screening fetal abnormalities in early pregnancy, early detection, early diagnosis, and early management can be achieved. At present, prenatal ultrasonography screening at 11~13+6 weeks has been carried out in some higher level hospitals and has received good clinical results, and will be more widely carried out and popularized to every pregnant woman in the near future. 1.2.1 Indications It is suitable for all pregnant women, especially those who have the following indications: pregnant women <18 years old or ≥35 years old, one of the spouses is a carrier of chromosomal balanced translocation, chromosomal abnormality in pregnant women, pregnant women suffering from diseases such as anemia, diabetes mellitus, hypertension, severe nutritional disorders, pregnant women who are smokers, alcoholics, history of X-ray irradiation or viral infections in early pregnancy, history of abnormal fetus, family history of genetic diseases, IVF and other diseases. family history of genetic disease, in vitro fertilization. 1.2.2 Examination contents (1) Fetal number and chorionicity. (2) Fetal heartbeat. (3) Measurement of head and hip length: the measurement should be made in the mid-sagittal section of the fetus, with the fetus in a natural position, without excessive backward or forward bending. The image should be enlarged as much as possible to show only the fetus. The skin contour lines of the top of the head and buttocks should be clearly shown. (4) Measurement of NT: It is recommended that the measurement be taken when the head-buttock length is 45-84 mm, which corresponds to 11-13+6 gestational weeks. The standard plane of measurement is the median sagittal view of the fetus, which is also the standard plane for measuring the head-buttock length. The image should be enlarged as much as possible to show only the fetal head, neck, and upper thorax, so that a slight movement of the measuring cursor can only change the measurement result by 0.1 mm. The characteristics of the standard NT measurement plane: the fetal face is clearly shown, with the skin line on the surface of the nasal bone, the nasal bone, and the tip of the nasal bone forming three short, strong echoes; the mandible is shown as a strong, rounded echogenicity; and the fetal cranium is clearly shown, with the thalamus, midbrain, brain stem, fourth ventricle, and postcranial fossa pool. The fetal cranium clearly shows the thalamus, midbrain, brainstem, fourth ventricle, and posterior fossa. The dorsal nuchal translucency is clearly visualized in the subcutaneous region of the back of the neck as the posterior nuchal translucency. The skin on the back of the neck and the two parallel hyperechoic bands anterior and posterior to the NT should be clearly visualized and confirmed. Measurements should be taken at the widest part of the NT and perpendicular to the strong echogenic bands of the skin, and the inner edge of the measuring cursor should be placed at the outer edge of the non-echoic NT. Multiple measurements should be taken and the maximum value obtained should be recorded. In the presence of cervical cerebrospinal bulge, pay attention to identification to avoid mismeasurement. In the presence of umbilical cord bypass, the thickness of the NT above and below the cord bypass should be measured and averaged.The NT value thickens with increasing gestational weeks, but generally does not exceed 3.0 mm.The risk of fetal chromosomal abnormality is increased with a thickened NT. A clear distinction should be made between skin and amniotic membrane to avoid mistaking amniotic membrane for skin and measuring NT incorrectly.(5) Pulsed Doppler detection of venous catheterization spectrum: Zoom in on the image in the median sagittal plane to show only the lower thorax and upper abdomen of the fetus. The angle between the sound beam and the venous catheter flow is adjusted so that the angle is less than 60° if possible. The pulsed Doppler sampling volume should be adjusted to the venous catheter blood flow signal, not exceeding the size of the venous catheter if possible. (6) Fetal appendages: Observe the position of the placenta and measure its thickness. Measure the maximum depth of the amniotic pool. (7) Pregnant woman's uterus: observe the endocervical os, if the pregnant woman provides a history of uterine fibroids, the location and size of the fibroids should be assessed. Obstetric ultrasound examination in middle and late pregnancy 2.1 Class III obstetric ultrasound examination 2.1.1 Indications Suitable for all pregnant women, especially suitable for pregnant women with the following indications: general prenatal ultrasound examination (Class I) or routine prenatal ultrasound examination (Class II) to find out or suspected fetal malformations, and high-risk factors for fetal malformations. 2.1.2 Examination contents 2.1.2.1 Number of fetuses Multiple pregnancies, the number of amniotic sacs should be clarified. 2.1.2.2 Fetal orientation Fetal orientation needs to be reported after 28 weeks of gestation. In addition to reporting the fetal orientation of each fetus in a multiple pregnancy, the positional relationship between the fetuses, e.g., left side of the uterine cavity, right side of the uterine cavity, upper uterine cavity, lower uterine cavity, needs to be indicated. 2.1.2.3 Fetal heartbeat A normal fetal heart rate of 120-160 beats/min. fetal arrhythmia, or a sustained heart rate of >160 beats/min or a sustained heart rate of <120 beats/min should be recommended for a fetal echocardiogram. 2.1.2.4 Biological Measurements A. Biparietal Diameter: Biparietal diameter should be measured in a standard thalamic level cross-section. The standard thalamic horizontal cross-section requires an oval, strongly echogenic ring in the skull, symmetry of the two cerebral hemispheres, and a centered midline, clearly showing the septum pellucidum, symmetrical thalamus on both sides, and the fissure-like third ventricle between the thalamus. When measuring the biparietal diameter, the measuring vernier is placed from the outer edge of the proximal skull to the inner edge of the distal skull, and is perpendicular to the midline of the brain. If the fetal head is too flat or too round, the error in estimating the gestational week using the biparietal diameter is large, and the head circumference should be added. The head circumference and biparietal diameter are measured in the thalamus level cross-section, and the measuring vernier is placed at the outer edge of the strong echogenic ring of the skull when measuring the head circumference.B. Transverse cerebellar diameter: The transverse cerebellar diameter should be measured in the cerebellar level cross-section. A standard transverse cerebellar view should show both the cerebellar hemispheres clearly and symmetrically, as well as the anterior septum pellucidum.C. Humerus/femur length: To show the long axis of the humerus/femur in cross-section, it is preferred that the acoustic beam should be perpendicular to the long axis of the humerus/femur, or that the beam be pinched at an angle of 45-90° to the humerus/femur, and that the humerus/femur ends be clearly visualized, and that the measurement vernier be placed at the midpoint of each end of the humerus/femur, excluding the humeral/femoral epiphyses. femoral epiphysis. After 14 weeks of gestation, the femoral length is more reliable for estimating the week of gestation. d. Abdominal circumference: Abdominal circumference should be measured in a standard epigastric cross-section. The standard epigastric cross-section is subcircular, with the liver and stomach visible, the umbilical vein connected to the left portal vein, the kidneys are not shown, and the spine cross-section reveals three strongly echogenic masses, with the measuring vernier placed at the outer edge of the skin. When there is a large umbilical bulge, abdominal cleft, or a large amount of ascites, the error of using abdominal circumference to estimate the week of pregnancy is large, and the use of abdominal circumference to estimate the week of pregnancy should be abandoned.E. Ultrasound assessment of the week of pregnancy and body weight: ultrasound assessment of the week of pregnancy and body weight is calculated by ultrasound measurement of the biparietal diameter, abdominal circumference, and the length of the femur, which are all subject to error. The error range of ultrasound weight estimation is generally ±15%; the error of ultrasound estimation of gestational week is small before 26 weeks of pregnancy, while the error is larger after 26 weeks, which is about ±2 to 3 weeks. Measurement and sectioning errors are present in ultrasound assessment of gestational weeks and body weight, and even if the same examiner takes multiple measurements during a single examination or different examiners take measurements during a single examination, the results will not be identical. Ultrasound review to assess fetal growth rate is often scheduled after 2 to 4 weeks. 2.1.2.5 Examination of Fetal Anatomical Structures A. Fetal head: requires observation of the skull, brain, cerebral falx, pellucid septum, thalamus, third ventricle, lateral ventricles, cerebellar hemispheres, cerebellar earthworms, and the pool of the posterior cranial fossa. The following 3 sections are important for the display and observation of these contents: thalamus horizontal transverse section, lateral ventricle horizontal transverse section and cerebellum horizontal transverse section.B. Fetal Face: It is required to observe the fetal double orbits, double eyeballs, nose and lips. The following three views are important for the visualization and observation of these elements: horizontal transverse section of both eyeballs, coronal section of the nose and lips, and median sagittal section of the face.C. Fetal neck: requires the observation of fetal neck masses, skin edema, and hydatid cystic tumors.D. Fetal thorax: requires the observation of both lungs, and the cardiothoracic ratio of the fetus.E. Fetal chest: requires the observation of both lungs, and the cardiothoracic ratio of the fetus. The following views are important for the visualization and observation of these structures: transverse section of the chest (four-chambered heart view). e. Fetal heart: requires observation of the fetal cardiac axis, apical pointing, atria, ventricles, interatrial septum, interventricular septum, atrioventricular valve, aorta, and pulmonary artery. The following views are important for the visualization and observation of these elements: four-chamber view of the heart, left ventricular outflow tract view, right ventricular outflow tract view, three-vessel view, and three-vessel tracheal view.F. Fetal diaphragm: Requires the observation of the diaphragm's continuity, abdominal organs (gastric vesicles, liver, etc.), and the position of the heart in relation to the diaphragm. The following views are important for the visualization and observation of these structures: coronal view of the diaphragm (or sagittal views of the left and right diaphragm, respectively).G. Fetal abdomen: requires observation of the liver, stomach, both kidneys, bladder, intestines, and the entrance of the umbilical cord into the abdominal wall. The following views are important for the visualization of these elements: transverse view of the upper abdomen, transverse view of both kidneys (or sagittal view of the left and right kidneys or coronal view of both kidneys, respectively), transverse view of the bladder at the level of the umbilical artery, and transverse view of the umbilical cord at the entrance to the abdominal wall. h. Fetal spine: requires the visualization of cervical, thoracic, lumbar, and sacrococcygeal segments of the spine. The following sections are important for the display and observation of these elements: sagittal section of the spine is routinely displayed, and coronal and transverse sections of the spine can be added when spinal abnormality is suspected.I. Fetal limbs: It is required to observe the bilateral upper arms and their inner humerus, bilateral forearms and their inner ulna and radius, bilateral thighs and their inner femur, bilateral lower legs and their inner tibia and fibula, and the presence of both hands and both feet. The following views are important to show and observe these: left and right humeral long-axis views, left and right ulnar and radial long-axis views, left and right ulnar and radial short-axis views, left and right femoral long-axis views, left and right tibial and fibular long-axis views, left and right tibial and fibular short-axis views, and sagittal/coronal views of both hands/feet. 2.1.2.6 Placenta Requires observation of placental position, maturity, relationship of the lower edge of the placenta to the internal cervical os, umbilical cord placental inlet, and measurement of placental thickness, which should be measured as the maximum vertical distance between the maternal and fetal sides of the placenta. The following views are important to show and observe these elements: umbilical cord placental inlet view, placental thickness measurement view, and sagittal view of the endocervical opening. Placenta praevia is not usually diagnosed before 28 weeks of gestation. When the umbilical cord placental inlet is difficult to show or is not shown, this should be noted on the report. Placental abruption is mainly a clinical diagnosis and its prenatal ultrasound detection rate is low, reported to be about 2% to 50%. 2.1.2.7 Umbilical cord The number of umbilical cord vessels, the placental entrance to the umbilical cord and the entrance to the fetal abdominal wall, and the umbilical artery blood flow spectrum assessed at 28 weeks are required to be visualized. The following views are important to show and observe these elements: transverse section of the bladder at the level of the umbilical artery, section of the umbilical cord placental inlet, and section of the umbilical cord abdominal wall inlet. 2.1.2.8 Amniotic fluid volume Amniotic fluid volume is assessed using the maximum depth of the amniotic pool or the amniotic fluid index. When measuring the maximum depth of the amniotic pool, the ultrasound probe should be perpendicular to the horizontal plane. There should be no umbilical cord or limb in the measurement area. The amniotic fluid index is measured by dividing the abdomen into 4 quadrants centered on the umbilicus of the mother, and the maximum depth of the amniotic pool in each quadrant is measured and summed to determine the amniotic fluid index. 2.1.2.9 Maternal uterus and bilateral adnexa Observation of the uterine wall, cervical canal, endocervical os, and bilateral adnexa is required. When the sagittal view of the cervix is not clear on transabdominal ultrasound, further transperineal ultrasound or transvaginal ultrasound is required. Transvaginal ultrasound is best for visualization of the endocervical os, but is contraindicated in the following conditions: cervical insufficiency, active vaginal bleeding, and vaginitis. Care should be taken to sweep the uterine wall to detect larger fibroids when possible and to observe both adnexa. There is insufficient evidence to support the widespread use of Doppler to observe uterine artery flow in low-risk populations, but measurement of uterine artery flow spectrum is recommended when fetal growth restriction (FGR) or hypertensive disorders of pregnancy are suspected. 2.1.3 Precautions 2.1.3.1 It is unlikely that all fetal anomalies can be detected by systematic prenatal ultrasonography Although systematic prenatal ultrasonography (grade III) provides systematic screening of fetal anatomy, and the main anatomical structures of the fetus can be visualized and displayed through the abovementioned views, it is unrealistic and impossible to expect that all fetal anomalies can be detected by systematic prenatal ultrasonography. The prenatal ultrasound detection rates of some fetal malformations reported in domestic and international literature are as follows for reference. (1) Anencephaly: more than 87%. (2) Severe brain enlargement: more than 77%. (3) Open spina bifida: 61%~95%. (4) Severe thoracic and abdominal wall defects with visceral ectopia: 60%~86%. (5) Fetal cleft lip and palate: 26.6%~92.54%. (6) Simple cleft palate: 0-1.4%. (7) Diaphragmatic hernia: about 60.0%. (8) Atrial septal defect: 0-5.0%. (9) Ventricular septal defect: 0~66.0%. (10) Hypoplastic left heart syndrome: 28.0%~95.0%. (11) Tetralogy of Fallot: 14.0%~65.0%. (12) Right ventricular double outlet: about 70.0%. (13) Prenatal ultrasound detection rate of single arterial trunk: around 67.0%. (14) Gastrointestinal malformation: 9.2%~57.1%. (15) Fetal limb malformations: 22.9%~87.2%. 2.1.3.2 Systematic prenatal ultrasonography (Grade III) is affected by some potential factors, such as the fat thickness of the abdominal wall of the pregnant woman, which may lead to acoustic attenuation and poor image quality; certain positions of the fetus may affect the observation of some parts of the body (e.g., it is difficult to show the face of the fetus in the occipital anterior position, it is difficult to show the heart, and it is difficult to show the face of the fetus when it is close to the wall of the uterus, etc.); when there is too much amniotic fluid, the fetus moves around too much, which makes it difficult to get a standard cut; and when there is too little amniotic fluid When the amniotic fluid is too much, it is difficult to obtain a standardized view; when the amniotic fluid is too little, it is difficult to show the fetal structure due to the lack of good amniotic fluid. Therefore, when it is difficult to complete all the required examinations in one ultrasound examination, the pregnant woman should be informed and prompted on the examination report, and recommended to review or referral. 2.1.3.3 Timing of systematic prenatal ultrasonography (Class III) It is recommended to be performed at 20 to 24 weeks of pregnancy. 2.2 Class II obstetric ultrasonography 2.2.1 Indications (1) Preliminary screening for the six major categories of lethal malformations stipulated by the Ministry of Health: anencephaly, severe brain enlargement, severe open spina bifida, severe thoracic and abdominal wall defect with visceral ectopia, single-cavity heart, and lethal chondrodysplasia. (2) Estimation of gestational week and assessment of fetal growth. (3) Loss of fetal movement, determination of fetal orientation, suspicion of ectopic pregnancy, suspicion of abnormal amniotic fluid volume, preoperative fetal head inversion, premature rupture of membranes, vaginal bleeding, lower abdominal pain. 2.2.2 Examination contents In addition to completing the contents of Level I obstetric ultrasonography, screening for the six major categories of severe structural malformations prescribed by the National Health and Family Planning Commission should be screened. The specific contents and requirements of each item are the same as those of Level III obstetric ultrasound examination if not specifically stated. (1) Fetal number, fetal heartbeat, fetal orientation, and placenta (Only placental position, thickness and maturity are required to be assessed. Placental thickness should be measured as the maximum vertical distance between the maternal and fetal sides of the placenta), and amniotic fluid volume. (2) Biological measurements (biparietal diameter, head circumference, femur length, abdominal circumference, ultrasound assessment of gestational week and weight). (3) Maternal uterus and both adnexa. (4) Examination of fetal anatomy: A. Fetal skull: It is required to observe the integrity of the skull, brain tissues and the posterior cranial fossa pools, and the following sections are important for the display and observation of these contents: thalamus horizontal cross-section, cerebellum horizontal cross-section. b. Fetal heart: It is required to observe the atria, ventricles, atrial septum, and atrioventricular valves, and the following sections are important for the display and observation of these contents: four-chambered heart section. c. Fetal spine. D. Fetal abdomen: requires observation of the abdominal wall, liver, stomach, both kidneys, bladder, and number of umbilical arteries. The following views are important to visualize and observe: transverse view of the upper abdomen, transverse view of the umbilical cord insertion through the abdominal wall, transverse view of the bladder at the level of the bladder, and transverse or sagittal or coronal views of both kidneys. e. Fetal limbs: requires observation and visualization of the femur on one side, and measurement of the femur length. The following views are important for the display and observation of these elements: left or right femur long axis view. 2.2.3 Precautions (1) Routine prenatal ultrasound examination at 20 to 24 weeks of gestation (Class II) should screen for the six major categories of lethal malformations specified by the National Health and Family Planning Commission. At present, the prenatal ultrasound detection rate of these malformations is not 100% as reported in domestic and international literature, for details, please refer to the precautions for Level III examination. (2) Routine prenatal ultrasound examination (Grade II) should at least examine the above fetal anatomical structures. However, sometimes, due to fetal position, low amniotic fluid, and maternal factors, the ultrasound examination does not show these structures well, and the ultrasound report should indicate this. 2.3 Class I obstetric ultrasonography 2.3.1 Indications Estimation of gestational week, assessment of fetal size, determination of fetal orientation, suspicion of ectopic pregnancy, disappearance of fetal movement, suspicion of abnormal amniotic fluid volume, preoperative head reversal, premature rupture of the membranes, assessment of placental position and placental maturity. 2.3.2 Examination contents The specific contents and requirements of each item are the same as those of Level II obstetric ultrasound examination if not specifically stated. Fetal number, fetal heartbeat, fetal orientation, placenta, amniotic fluid volume, biological measurements (biparietal diameter, femur length, abdominal circumference, ultrasound assessment of gestational week and weight). 2.3.3 Precautions The general prenatal ultrasound examination (Level I) mainly performs examination of the main fetal growth parameters, not examination of fetal anatomy and structure, and does not screen for fetal anomalies. If the examining physician finds fetal abnormalities, the ultrasound report should provide specific instructions and referral or recommendation for systematic prenatal ultrasound examination (Grade III). 2.4 Targeted prenatal ultrasound examination (Grade Ⅳ) Examinations for specific purposes for special problems of the fetus and the pregnant woman, such as fetal echocardiography, fetal neurological examination, fetal limb examination, and fetal facial examination. General prenatal ultrasonography (Grade I), routine prenatal ultrasonography (Grade II), and systematic prenatal ultrasonography (Grade III) Targeted prenatal ultrasonography (Grade IV) can be performed if fetal abnormalities are detected or suspected, if there are high-risk factors for fetal abnormality, or if there are abnormalities in maternal blood biochemical tests. 2.5 Limited prenatal ultrasonography Limited prenatal ultrasonography is mainly used for emergency ultrasonography or bedside ultrasonography, due to the critical condition or the difficulty of the pregnant woman to cooperate with the examination, and only examines the clinician's request to understand a specific problem, such as only understanding the number of fetuses, or fetal heart rate, or the pregnant woman's cervix, or the amount of amniotic fluid, or the fetal position, or the pelvic and abdominal cavity effusion and so on.