Almost all internal organs of the fetus will continue to grow and change before birth, therefore, it is not guaranteed that the organs that are normal at the early examination will be normal after birth. Ultrasound is also limited by the obstruction of the mother’s belly and uterus, and cannot adjust the position of the fetus to obtain images from certain angles. The following are the conditions that limit the ultrasound examination due to the growth of the fetus: 1. The fetus can be easily diagnosed at birth, but there is no way to know whether the fetus has congenital hearing impairment; 4. Abnormalities of the extremities (fingers and toes), such as inversion or ectropion of the hands and feet, polydactyly, syndactyly, or missing knuckles, can hardly be diagnosed by ultrasound because the fetus is often in a fist-clenching state; 7. Most of the biochemical metabolic abnormalities, such as mucopolysaccharidoses, will not develop until after the baby is born and fed. Therefore, many lethal biometabolic abnormalities cannot be recognized before birth unless the mother has already given birth to a baby with a similar disease. The first ultrasound is performed in the 18th-20th weeks. It provides a general diagnosis of fetal malformations and observes the state of fetal activity. It gives a quick “answer” to highly suspected fetuses such as anencephaly, hydrocephalus, cerebrospinal bulge, umbilical cord anomaly, digestive tract anomaly, conjoined anomalies, microcephaly, etc. The purpose of the second ultrasound in the 34th week of pregnancy is to monitor the amount of amniotic fluid, the position of the placenta, the maturity of the placenta and the presence of fetal anomalies, and to find out whether the fetal development corresponds to the gestational week. Finally, after the 37th week, the ultrasound will provide the main basis for determining the mode of delivery, in addition to understanding the position of the placenta, the presence of fetal malformations, and whether the fetal development corresponds to the gestational week. In order to monitor the amniotic fluid volume and placental maturity, weekly ultrasound is needed if necessary.