The secondary ultrasound for pregnant women is usually performed between 20-24 weeks of pregnancy, with the optimal period being between 22-24 weeks and generally not more than 26 weeks. It is important to consult the obstetrics department of a regular hospital to request this examination and choose the specific examination items according to the doctor’s recommendations. During the 20-24 weeks of pregnancy, all parts of the fetus have basically developed, and a secondary ultrasound can detect whether the fetus is malformed, as well as perform systematic examinations of the fetus, such as whether the spine is neatly arranged, the size and shape of the liver, whether the limbs are clear, the size and shape of the fetal heart, whether the septum is continuous, whether both kidneys are clear, and whether there is fluid accumulation. The examination is more important at this time, and if abnormal results appear, it can be combined with other indicators of pregnancy for comprehensive judgment and further prenatal diagnosis if necessary. If the abnormality is found late, the best time for treatment may be missed, threatening the life of the fetus. For some cases of fetal abnormalities, observation can be continued and further amniocentesis can be done if needed for fetal chromosome examination as well as diagnosis, which can avoid missed diagnosis. If severe fetal abnormalities are found, the pregnancy needs to be terminated in time.