Fetal development in the uterus is a continuous process, and ultrasonography at different trimesters can detect different types of fetal abnormalities, but not all fetal structural malformations. And some manifestations of fetal malformations can be detected only during specific gestational weeks. Some ultrasound soft markers associated with chromosomal abnormalities, such as choroid plexus cysts and renal pelvic dilatation, may disappear as the pregnancy progresses. Therefore, the period of prenatal fetal ultrasound examination is very important, and if the optimal period is missed, it will inevitably lead to uncertainty in the examination results. It is crucial to choose the appropriate timing, frequency, content and method of ultrasound examination. According to the norms of prenatal ultrasonography, it is appropriate to conduct 4~6 ultrasonography examinations during the whole pregnancy (except for special circumstances), and their corresponding time periods are as follows: Routine ultrasonography in early pregnancy: it is carried out at about 6~8 weeks of menopause. In this period, it mainly determines whether the pregnancy is intrauterine, the location and number of gestational sacs, evaluates the gestational week, whether there are heart beats, diagnoses multiple pregnancies, and excludes pregnancy-related abnormalities (ectopic pregnancy, hyperemesis gravidarum, embryonic stasis) and other gynecological disorders (pelvic masses, uterine malformations), and so on. In the case of multiple pregnancies, early pregnancy is also a critical period for determining chorionicity and amnionicity. 11~14 weeks of pregnancy: the main organs of the fetus have basically formed by the 12th gestational week, at this time the ultrasound can show some of the main systems and organ structures of the fetus, focusing on the measurement of the fetal nuchal translucency (NT), nasal bone, and so on. It also further determines the number of fetuses and the chorionicity of twin pregnancies. It is a good time to determine the week of pregnancy and recommend for pregnant women who are undergoing early diagnosis of chromosomal disorders such as Down’s screening. Middle gestation period (18~24 gestational weeks): During this period, the fetal organs are basically mature, amniotic fluid volume is moderate, and the ultrasound image is clear, which is the best time for fetal malformation screening. We believe that 22~24 gestational weeks is the best. 32 weeks of gestation~pre-birth: 1~2 ultrasound examinations can be performed in this period. It is already late in pregnancy, focusing on assessing fetal position, fetal growth and development, placenta, amniotic fluid, umbilical cord blood flow, and the presence of umbilical cord around the neck. If conditions permit, fetal malformations that have not yet appeared in mid-pregnancy (late malformations) can be detected.