According to the “Guangdong Provincial Department of Health Prenatal Diagnostic Technology Management Implementation Rules” and the spirit of relevant supporting documents, prenatal ultrasound examination is divided into three categories (implemented since September 1, 2006): I. Prenatal ultrasound examination (i.e. prenatal general ultrasound examination): including early pregnancy and prenatal ultrasound examination of middle and late pregnancy. 1.Adapted to hospitals that have not obtained the qualification of prenatal ultrasound screening and prenatal ultrasound diagnosis, or have obtained the qualification of prenatal ultrasound screening and prenatal ultrasound diagnosis, but have received level III or level II prenatal ultrasound examination. 2. Examination content: mainly check fetal growth related parameters (gestational sac position size, germ length, biparietal diameter, abdominal circumference, femur length, placenta position and maturity, amniotic fluid volume, fetal heart rate and umbilical artery blood flow parameters, etc.). 3. No screening or diagnosis of fetal malformation will be performed. Prenatal ultrasound examination (i.e. prenatal screening by ultrasound): including fetal ultrasound examination in middle and late pregnancy, which is more suitable to be performed in 20~24 weeks of pregnancy. 1.Adapted to hospitals and personnel who have obtained the qualification of prenatal ultrasound screening. 2, examination content: In addition to the content of prenatal ultrasound examination of grade I, it should include: morphological examination of major fetal organs, such as observation of certain important intracranial structures, four-chambered heart section, liver, stomach, kidney and other organs in the abdominal cavity, cursory screening of serious fetal lethal malformations (anencephaly, severe brain expansion, severe open spina bifida, severe chest and abdominal wall defects and visceral ectopia, single-chambered heart, fatal chondrodysplasia). 3. If the fetus is found to have suspicious developmental abnormalities or malformations, it should be referred to a hospital qualified for level III prenatal ultrasound for prenatal ultrasound diagnosis. Level III prenatal ultrasonography (also known as systemic fetal ultrasonography or prenatal ultrasonography diagnosis): including fetal ultrasonography in middle and late pregnancy and fetal ultrasonography when fetal malformation is found or suspected or when there are high-risk factors for fetal malformation, it is more appropriate to perform systemic fetal ultrasonography in 20~24 weeks of gestation, if fetal malformation is found or suspected by ultrasonography, prenatal ultrasonography diagnosis should be performed in any gestational week. 1.Adapted to hospitals and personnel who have obtained the qualification of prenatal ultrasound diagnosis. 2.Examination content: In addition to the content of level II prenatal ultrasound, the examination content is more in-depth and detailed (e.g. fetal heart includes left and right ventricular outflow tract in addition to four-chamber heart). 3. The fetus should receive a level III prenatal ultrasound examination (systemic fetal ultrasound examination) at 20~24 weeks if possible. Special note: Prenatal ultrasonography cannot detect all fetal malformations. Fetal anomalies that should be detected by ultrasound at 20 to 24 weeks of gestation include anencephaly, severe brain expansion, severe open spina bifida, severe abdominal wall defect and visceral exostosis, single-chambered heart, and fatal chondrodysplasia. The higher the level of prenatal ultrasound, the more types of fetal malformations can be detected.