Generally speaking, ultrasound examination during pregnancy can be performed in four stages. 10-14 weeks of pregnancy: Ultrasound examination can be done when the pregnancy has unexplained vaginal bleeding, lower abdominal pain, suspicion of gravidity and other abnormalities; for those who do not have any clinical symptoms, this examination can be temporarily excluded. 20-24 weeks of gestation: Ultrasound examination at this time can understand the development of fetal limbs and heart. At this stage, the fetal limbs are fully differentiated and the amount of amniotic fluid is relatively high, so the display rate is higher. 27-32 weeks of gestation: At this stage, the fetal facial structure can be observed; the observation of the fetal heart is the focus; it can make a clearer diagnosis of some more serious fetal malformations. Full-term pregnancy stage (after 28 weeks) This is the best time to examine the internal organs of the fetus; 36 weeks is the most suitable period for comprehensive examination of the fetus and its appendages. In some special cases, ultrasound examinations should be performed at any time or periodically, regardless of the stage of pregnancy. For example, suspicion of ectopic pregnancy; abnormal laboratory tests (elevated or decreased methemoglobin, increased free estriol, etc.); significant teratogenic factors (intake of potentially teratogenic drugs, exposure to radiation, etc.); history of hereditary diseases or family genetic disorders; history of infection during pregnancy (rubella virus, cytomegalovirus infection, etc.); history of previous pregnancies with births of structurally malformed fetuses (congenital systemic heart disease, etc.); pregnant women Diabetes or other diseases; suspected fetal death; uterine size does not correspond to the gestation period; suspected intrauterine growth retardation; suspected fetal malformations, abnormalities of amniotic fluid, placenta or umbilical cord; considering twin or multiple pregnancies; estimation of gestational age in cases of irregular menstruation; understanding fetal growth and development; assessment of fetal weight, etc.; physical evaluation of the intrauterine state of the fetus; determination of fetal previa, fetal position; diagnosis of cervical maturity; Follow-up and observation of normal or abnormal fetal conditions, etc.