With the continuous development of ultrasound diagnostic technology and ultrasound instruments and the accumulation of clinical experience, ultrasound examination during pregnancy has become an important means to evaluate fetal growth and development, detect fetal structural abnormalities and screen fetuses at high risk for chromosomal abnormalities. Since the implementation of the “Measures for the Administration of Prenatal Diagnostic Technology” of the Ministry of Health, prenatal ultrasound diagnosis has been developed more rapidly, while the concept of eugenics and eugenics among pregnant women has been increasing, and they have more knowledge and higher requirements for prenatal fetal ultrasound examination. In China, the population is large, the geographical area is vast, and the differences between urban and rural areas are large, so it is difficult to unify the timing, content, instruments, requirements of physicians’ technical level and charges for prenatal ultrasound examination, and the detection rate of fetal malformations detected by ultrasound still varies greatly in different regions and hospitals. For this reason, according to the management methods of prenatal diagnosis technology promulgated by the Ministry of Health and with reference to the “Guidelines for prenatal ultrasound examination”, the domestic professional ultrasound association has prepared the “Standard for prenatal ultrasound examination”, which is a specialized standard and document for the ultrasound profession, aiming to standardize and improve the quality of obstetric ultrasound examination, and is a general standard and principle to be followed for obstetric ultrasound examination. It is the general standard and principle that should be followed. According to the standard of prenatal ultrasound examination, obstetric ultrasound examination can be divided into three categories: ① Routine prenatal ultrasound examination: including early pregnancy and general ultrasound examination of middle and late pregnancy. It is mainly used to evaluate embryonic and fetal growth and development, placenta, amniotic fluid and other conditions. ②Systematic prenatal ultrasonography: including fetal system ultrasonography performed in early and middle pregnancy from 11 to 14 weeks of gestation and from 18 to 24 weeks of gestation. The purpose is to screen for fetal malformations. ③Targeted examination. In clinical work, due to the specific condition of pregnant women and ultrasound equipment, there are still inevitably some deviations or exceedances, and, in recent years, ultrasound examinations during pregnancy have become increasingly early and multiple, therefore, the reasonable choice of timing, number and process of fetal ultrasound examination and the safety of its ultrasound examination has become a concern for clinicians and pregnant women. The development of the fetus in utero is a continuous process, and its ultrasound images have different characteristics in different developmental periods. Different types of fetal abnormalities can be detected by ultrasound examination in different trimesters, but not all fetal structural malformations can be detected. As the fetal organs develop, their hidden malformations or abnormalities are gradually revealed, and some fetal malformation manifestations can only be detected in a specific gestational week. Some ultrasound soft markers associated with chromosomal abnormalities, such as choroid plexus cysts and dilated renal pelvis, may disappear as pregnancy progresses. Therefore, the period of prenatal fetal ultrasound examination is very important, each period has a different focus of observation, and once the optimal period is missed, it will inevitably lead to uncertainty in the results. In order to maximize the early detection of fetal malformations and reduce missed diagnoses, so as to facilitate early clinical decision-making and reduce the burden on pregnant women and society, it is crucial to choose the appropriate timing, frequency, content and method of ultrasound examination. According to the standard of prenatal ultrasound examination, combined with the specific medical situation in China, we believe that it is appropriate to perform 4-6 ultrasound examinations throughout pregnancy, and the corresponding time periods are: 1.1 Early pregnancy routine ultrasound examination: a routine ultrasound examination should be performed at about 6-8 weeks of menopause. During this period, the main purpose is to determine whether the pregnancy is intrauterine, the location and number of gestational sacs, assess the gestational week, the presence of heartbeat, diagnose multifetal pregnancy, exclude pregnancy-related abnormalities (ectopic pregnancy, gravida, embryonic abortion) and other gynecological disorders (pelvic masses, uterine malformations), etc. For multiple pregnancies, the early pregnancy period is also a critical period to determine their chorionicity and amniotic nature. 1.2 Systematic ultrasound screening during early pregnancy (11-14 gestational weeks): Fetal genetic ultrasound can be performed during this period to screen for fetuses at high risk for chromosomal abnormalities. Emphasis is placed on measuring the nuchal translucency (NT), nasal bone and, if available, the venous catheter and tricuspid valve blood flow spectrum. The major organs of the fetus are basically formed at 12 weeks of gestation. Ultrasound at this time can reveal the structures of the major fetal systems and organs, thus allowing the detection of some fetal structural malformations for early diagnosis and early decision making. At this stage, the fetal head and intracranial structures, facial orbits, continuity of the spine, four-chambered heart, stomach, bladder, heart, stomach, bladder, integrity of the abdominal wall, and movement of the extremities are mainly observed. Also further determine the number of fetuses, chorionicity of twin pregnancies. Determine the week of gestation and recommend a good time for the pregnant woman who will have Down’s syndrome screening. 1.3 Systematic ultrasound screening in the middle trimester (18-24 weeks): the fetal organs are basically mature, the amniotic fluid volume is moderate, and the ultrasound image is clear, which is the best time for fetal anomaly screening, and most fetal structural anomalies can be detected at this stage. In addition to the routine evaluation of fetal growth parameters, amniotic fluid, placenta, gestational number and fetal position, a comprehensive and detailed systematic ultrasound screening should be performed. We believe that 22 to 24 gestational weeks is the best. The detection rate of fetal malformations can be affected by the condition of the instrument, technical experience of the examiner, fetal size, skeletal acoustic shadow, amniotic fluid volume, fetal position, thickness of the abdominal wall of the pregnant woman, and scarring. Ultrasound screening in early pregnancy and mid-pregnancy have their own advantages and cannot replace each other. However, if the conditions are limited or the pregnant woman delays the ultrasound screening period in early pregnancy (11-14 weeks), it is especially important to perform a systematic ultrasound examination in the middle 18-24 weeks of pregnancy to screen for fetal structural malformations. If abnormal signs of fetal structure are found or suspected during the ultrasound examination, it should be examined in detail for the presence of other systemic and organ abnormalities. In addition to systematic fetal ultrasound, careful targeted ultrasound examinations can be performed to address the specific problems or requirements of the fetus or the pregnant woman. If necessary, chromosomal examination and referral to higher level hospitals and prenatal diagnosis centers for further ultrasound diagnosis should also be done. If systemic ultrasound in midtrimester reveals only one or more ultrasound soft signs, such as thickened skin folds (NF) in the neck, choroid plexus cysts, intestinal or intracardiac strong echogenicity, single umbilical artery, etc., without a combination of serious malformations, individual risk assessment for chromosomal abnormalities should be considered first, and fetal echocardiography may be recommended. 1.4 Routine ultrasound examination from 28 to 30 weeks of gestation: This period is already late in pregnancy and focuses on assessing fetal growth and development, placental position, amniotic fluid, fetal position, etc. Although the purpose of routine prenatal ultrasonography is to observe the growth and development of the fetus and is mostly used for pregnant women who have had one fetal system ultrasonography, if resources permit, it is still recommended to perform morphological observation of important fetal organs at the same time to detect fetal malformations that have not yet appeared or may be missed in the middle of pregnancy. The fetal structures that were not observed satisfactorily during the 18-24 weeks of gestation ultrasound screening should also be reviewed. 1.5 Routine ultrasound examination at 34-38 weeks of gestation: 1 or 2 routine prenatal ultrasound examinations are recommended during this period. In addition to determining fetal development, amniotic fluid volume, placental maturity, umbilical cord blood flow, presence of umbilical cord encirclement and fetal position, ultrasound examination of important internal organs during this period is also of great clinical value and may detect some late fetal anomalies that appear late on ultrasound images, such as small umbilical bulge, hydronephrosis, hydrocephalus and fetal anomalies. hydrocephalus, hydrocephalus, etc. Although ultrasonography is currently the most effective method to screen for fetal structural malformations, there is a wide variety of fetal malformations and a significant number of malformations that cannot be detected or identified by ultrasound. In particular, ultrasound diagnosis of some birth defects without obvious morphological changes is still difficult and cannot replace all prenatal diagnostic techniques. A normal ultrasound examination also does not guarantee an absolutely normal fetal pregnancy outcome. Clinicians and pregnant women should be aware of the purpose, detection rate and limitations of ultrasonography during pregnancy.