Some mothers-to-be think that they will see more clearly when the fetus is older when they have an ultrasound, but they may have overlooked another problem. That is, if the recommended examination time is missed, as the fetus grows up, his free space to move around in the amniotic cavity will become more and more narrow, and the chance of fetal position change will decrease. It may be difficult for doctors to find the right angle during the examination, and they may not be able to observe the morphological structures of certain organs, which may cause difficulties in the elimination of malformations. In the period of 18~24 weeks of pregnancy, the fetus’ important organs are basically fully developed, but the length is moderate, the amniotic fluid is more, and the fetal body is more active, so the ultrasonographer can obtain more images. Therefore, we remind all mothers-to-be that it is best not to miss the systematic ultrasound examination from 18 to 24 weeks of pregnancy. The primary purpose of ultrasound screening is to avoid or reduce the number of births of lethal malformations. What are lethal malformations? Examples include: anencephaly, open spina bifida, severe thoracoabdominal wall bifida with visceral exostosis, single-chambered heart, and lethal chondrodysplasia, among others. These malformations generally have a high natural loss; even if they can be born, they are difficult to survive; and with the current medical treatment, there is no effective treatment to cure and improve these diseases, so they are called lethal malformations.