The main difference between level 1, level 2 and level 3 ultrasound is the timing of the examination and the content of the examination for pregnant women. Level I ultrasound examination only requires the measurement of biparietal diameter, femoral length and abdominal circumference. It is mainly to determine whether the fetus is alive, the position of the placenta and the condition of the amniotic fluid. In addition to the content of level I ultrasound, level II ultrasound also requires morphological observation of the major organs of the fetus, such as certain important structures in the skull and brain, four-chamber heart section, liver, stomach and kidney in the abdominal cavity, and screening for serious fetal lethal malformations. The tertiary ultrasound examination is more demanding and should have strict requirements for the ultrasonographer, instrumentation and equipment, and the size of the gestational week at the time of examination. This ultrasound should be performed in hospitals with prenatal diagnostic qualifications and doctors who are qualified as prenatal ultrasonographers to perform this examination.