Generally speaking, five to six ultrasound examinations are needed during pregnancy, and the focus of each examination varies, and the format and description of the report varies from hospital to hospital, so the following is for reference only. The first ultrasound examination is usually performed between 6 weeks and 8 weeks of pregnancy, mainly to observe whether the pregnancy is intrauterine, singleton or multiple, whether there is a fetal heartbeat, and the length of the fetal germ (head-rump length CRL), which is mainly for the reference of archiving. It takes about 3-5 minutes and you need to hold urine for the examination. The second ultrasound: 11-13 weeks and 6 days, mainly to measure the fetal NT value, takes 5-10 minutes, need to hold urine for examination, and may ask the pregnant woman to walk around outside and receive examination again due to the poor position of the fetus. The third ultrasound examination is performed between 16-19 weeks to provide ultrasound data for the Down screening test and to estimate the risk rate. At this time, the description of biparietal diameter, head circumference, abdominal circumference, femoral length, amniotic fluid depth, fetal position, and placental position appears in the report. It takes 5-10 minutes and does not require holding the urine. The fourth ultrasound examination: during 22-24 weeks, this examination is mainly to detect whether the fetus is suffering from major malformation, the intracranial structures, heart structures, gastric vesicles, both kidneys, bladder, intestinal canal, long bones of the limbs all need to be examined in detail, it takes longer, about 15-30 minutes or more. There is no need to hold urine. When the fetus is not in a good position, it is necessary to walk properly and then receive another examination. The fifth ultrasound examination: 28-32 weeks, the same as the fourth examination, since some abnormalities are not detected at mid-pregnancy and can only be recognized as the fetus grows, this examination is the second screening examination and the report starts to show the amniotic fluid index, fetal weight and umbilical artery S/D value. This examination takes slightly less time than the fourth ultrasound examination. There is no need to hold urine, but when the fetus is not in a good position, it is necessary to walk appropriately and then receive the examination again. The sixth ultrasound: between 36 and 38 weeks, the main purpose is to measure the fetal biparietal diameter, head circumference, abdominal circumference and femoral length to estimate the fetal weight, in addition to the amniotic fluid index, fetal position, placental position and maturity to provide the clinical doctor with a basis for deciding the mode of delivery. The time required is 5-10 minutes. When it is necessary to observe the length of the cervical canal or the position of the lower edge of the placenta, it is necessary to hold urine for a moderate period of time (when the urge to urinate is evident). In addition, the length of the cervical canal (in pregnant women with cervical insufficiency and at risk of preterm labor) and the thickness of the lower uterine muscular layer (in pregnant women with a history of cesarean section) are also measured in late pregnancy according to the individual condition of the woman. The above are the routine ultrasound measurements during pregnancy in most hospitals in China, but when a pregnant woman is uncomfortable, the clinician may also temporarily request an ultrasound examination to observe the condition of the fetus. It should be noted that due to different ultrasound instruments and different measuring habits of doctors, repeated measurements of biparietal diameter, head circumference, abdominal circumference and femur length in the same patient within a short period of time may result in errors (e.g., the smaller the measurement, the smaller the measurement) and may cause unnecessary disturbance to the pregnant woman. Therefore, the above parameters are usually not measured again within a week from the last ultrasound, but only to provide information about the amniotic fluid index, S/D value, placenta and whether it is rounded.