Ultrasound can detect large and severe fetal malformations. Further ultrasound examination is needed when there is a high level of methemoglobin and a high risk of developing neural tube defects, which should be performed by a qualified ultrasonographer. In addition, the maternal weight, the thickness of the abdominal wall, the position of the fetus, and the resolution of the ultrasound instrument can affect the results. For Down’s syndrome, there are no relevant definite ultrasound indicators to exclude. However, there are some indicators that can suggest a high probability of Down syndrome in the fetus, such as short femur, single umbilical artery, intestinal malformation, congenital heart disease, etc. However, the absence of these signs does not mean that the child is not a child with Down’s syndrome, so the possibility of a child with Down’s syndrome cannot be absolutely excluded. Children with trisomy 18 usually have multiple severe malformations, which can mostly be detected by ultrasound.