According to our specific situation and clinical requirements, obstetric ultrasonography is divided into the following 4 levels (levels): 1. General obstetric ultrasonography (level 1): It mainly assesses the general growth and development of the embryo and fetus, and the examination content only requires the measurement of biparietal diameter, femoral length diameter and abdominal circumference to determine whether the fetus is alive, placental position, fetal position and amniotic fluid situation. This level of ultrasound examination is not an ultrasound examination for the purpose of detecting fetal malformation, but it can still provide some meaningful diagnostic information to the obstetric clinic, and is suitable for pregnant women who have already undergone systematic ultrasound examination in poorer primary hospitals or hospitals with better conditions. 2.Conventional obstetric ultrasonography (second level): In addition to the completion of general obstetric ultrasonography, morphological observation of major fetal organs should be performed, such as certain important structures in the skull, four-chamber heart section, observation of liver, stomach, kidney and other organs in the abdominal cavity, as well as cursory screening of serious fetal lethal malformations. The lethal malformations that should be diagnosed at 18-24 weeks of gestation as stipulated by the Ministry of Health Prenatal Diagnostic Technique Management Measures include anencephaly, severe brain expansion, severe open spina bifida, severe chest and abdominal wall defects, visceral exostosis, single-chamber heart, and lethal chondrodysplasia. 3.Systematic fetal ultrasound examination (level 3): this level of examination is very demanding and has strict requirements for ultrasonographers, instruments and equipment, time required for examination, examination content and size of pregnancy week at the time of examination. Not all ultrasound physicians and hospitals can perform this level of ultrasound examination. We recommend that Level 3 ultrasound examinations should be performed in hospitals with prenatal diagnostic qualifications and by ultrasonographers who are qualified in prenatal ultrasound diagnosis, but other hospitals are not excluded from performing Level 3 ultrasound examinations. Through this level of ultrasound examination, the purpose of improving the detection rate of fetal malformations, reducing the birth of seriously defective children and improving the quality of our population is achieved. The instruments used for systematic examination require high resolution and clear images, and we recommend that it is best to use high-grade color Doppler ultrasonography for examination. 4, targeted ultrasound examination (level 4): this level of examination includes fetal echocardiography. It is usually carried out on the basis of the first 3 levels of examination, and the detailed examination is carried out for a special requirement or purpose, and fetal echocardiography belongs to this category.