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. Because ultrasound is blocked by the mother’s belly and uterus, it is not possible to adjust the position of the fetus and obtain images from certain angles. The following are the conditions that limit ultrasound examinations due to fetal growth: 1. hydranencephaly There are many hydronephrosis or hydronephrosis conditions that gradually arise in the second trimester. 2. Total blindness The fetus does not open its eyes in the womb because there is no light stimulation, so it is impossible to diagnose congenital total blindness or microphthalmia. 3.Hearing By the time the fetus is 5-6 months old, hearing has developed, but there is no way to know whether the fetus has congenital hearing impairment. Congenital heart disease The atrial septum of the heart (i.e., foramen ovale) and the ductus arteriosus are gradually closed after birth, and although heart disease can be easily diagnosed at birth, it cannot be known before birth. 5. Gastrointestinal tract obstruction Very few obstructive lesions of the gastrointestinal tract appear before the 24th week of pregnancy, because the fetus rarely swallows amniotic fluid in the first trimester. Abnormalities of the limbs (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. Dwarfism There are some symptoms of dwarfism that cannot be diagnosed at an early stage. This is because the fetus gradually stops developing and growing bones when it is 6 to 7 months old. 8. Inborn metabolic abnormalities Most of the biochemical metabolic abnormalities, such as mucopolysaccharidoses, do 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.