There are many patients who have questions about the number and timing of ultrasound examinations during their visits, which are briefly described below: The number and timing of prenatal ultrasound examinations are recognized differently in different countries and regions. In some countries, prenatal ultrasound is routine, with an average of 2.8 ultrasound exams per pregnant woman. In maternal and child health centers, obstetric ultrasound is included as a routine prenatal ultrasound examination, and it is generally recommended that pregnant women undergo a fetal NT examination at 11-14 weeks, a detailed systematic screening for fetal malformations at 24-28 weeks, an additional screening ultrasound examination at 32 weeks, and an antenatal fetal weight estimation and biological score at 38 weeks. Ultrasound should be performed at any gestational week if one of the following indications is present: 1. twin or multiple pregnancies. 2. Positive laboratory findings, such as elevated or decreased alpha-fetoprotein, elevated free estriol, positive pregnancy-related proteins, etc. 3. Previous pregnancies with fetuses born with structural abnormalities, such as congenital heart disease, kidney disease, etc. 4.Parents with hereditary diseases or family history of heredity. 5.Parents with a history of infection during pregnancy, such as rubella, cytomegalovirus infection, etc. 6.The mother has diabetes or other diseases. 7.People with obvious teratogenic factors, such as having taken potentially teratogenic drugs, exposure to radiation, exposure to toxic substances, etc. 8.Suspected fetal death. 9.Suspected intrauterine growth retardation of the fetus. 10.Suspected abnormalities of amniotic fluid, placenta and umbilical cord. 11.Fetal previa and fetal orientation determination. 12.Estimation of gestational age in irregular menstruation. 13.Fetal growth, fetal weight assessment, etc. 14.Diagnosis of cervical maturity. 15.Localization of amniocentesis. 16.Uterine size does not correspond to the duration of pregnancy. 17.Pelvic masses. 18, Suspected ectopic pregnancy. 19, Biophysical scoring of intrauterine status of the fetus. 20, Follow-up observation to determine fetal malformation.