How many ultrasound examinations are appropriate during pregnancy? Generally speaking, five to six ultrasound examinations are needed during pregnancy, and the focus of each examination varies, and the report format and description of each hospital also vary. The first ultrasound examination is usually performed between 6 weeks and 8 weeks of pregnancy, mainly to observe whether the pregnancy is intrauterine, single 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 is necessary to hold urine for the examination. The second ultrasound: 11-13 weeks and 6 days, mainly to measure the fetal NT value, due to the poor position of the fetus, the pregnant woman will be asked to walk outside and be examined again. The third ultrasound: between 16-19 weeks to provide ultrasound data for the Down screening test and to estimate the risk rate. At this time, the report includes descriptions of biparietal diameter, head circumference, abdominal circumference, femoral length, amniotic fluid depth, fetal position, and placental position. The fourth ultrasound examination: During 22-24 weeks, this examination is mainly to detect whether the fetus is suffering from major malformations. It is necessary to do detailed examination of intracranial structures, cardiac structures, gastric vesicles, both kidneys, bladder, intestinal canal, and long bones of the limbs, and 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 anomalies 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 is performed between 36 and 38 weeks to measure the fetal biparietal diameter, cephalic circumference, abdominal circumference and femoral length to estimate the fetal weight, in addition to the amniotic fluid index, fetal position, placenta position and maturity to provide a basis for the clinical doctor to decide the mode of delivery. 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 moderately (if there is a clear urge to urinate). In addition, the length of the cervical canal (in cases of cervical insufficiency and preterm labor) and the thickness of the lower uterine muscular layer (in women with a history of cesarean section) are also measured in late pregnancy, depending on the individual condition of the woman. These are the routine ultrasound examinations during pregnancy in most hospitals in China, but when a pregnant woman is uncomfortable, the clinician may also request an ultrasound examination on an ad hoc basis 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 one week from the last ultrasound.