Despite the variety of imaging modalities available today, prenatal ultrasound is still the safest, reliable, non-invasive and repeatable test currently used to monitor fetal development. Generally speaking, there are 5 necessary ultrasound examinations during pregnancy, which pregnant women should do on time and cannot be missing. These 5 examinations are, in order: 7~8 weeks of pregnancy to determine intrauterine pregnancy with live fetus, 11~14 weeks of pregnancy for nuchal translucency, 20~24 weeks of pregnancy for fetal system screening, 32 weeks of pregnancy for fetal growth and development assessment, and 38 weeks of pregnancy and beyond (before delivery) for fetal growth and development assessment. Special attention should be paid to the two most important examinations in the whole pregnancy: 11~14 weeks of pregnancy and 20~24 weeks of pregnancy, which must be performed in hospitals with professional institutions and qualified in prenatal ultrasound diagnosis. Early pregnancy checkups are done at 7~8 weeks of pregnancy. It is necessary to continue the pregnancy only if it is first determined to be a live intrauterine pregnancy, and both transabdominal and transvaginal ultrasound routes are available. Transvaginal ultrasound images are clearer than transabdominal ultrasound and will not affect the fetus if performed correctly. Therefore, in cases where transabdominal diagnosis is difficult, transvaginal ultrasound is usually required for further clarification, such as when the germ does not show, or when the heart tube is not pulsating clearly. In contrast, ectopic pregnancy must be determined by transvaginal ultrasound. The nuchal translucency examination is performed between 11 and 14 weeks of pregnancy. The nuchal translucency is the thickness of the fluid accumulation in the subcutaneous tissue behind the neck of the fetus. This test must be limited to 14 weeks of gestation, which for ultrasound means a parietal rump length of less than 84 mm, because after 14 weeks of gestation in cases of cervical hyaline layer thickening, the thickening changes may gradually subside and the measurement is no longer meaningful. Systematic screening is performed from 20 to 24 weeks of gestation. This is the most comprehensive ultrasound examination during the entire pregnancy. The doctor will perform a detailed examination of the fetal organ morphology, including intracranial structures, face, heart, gastric vesicles, kidneys, bladder, spine, and extremities. Ultrasound observation can be limited by the fetal position, maternal abdominal wall obesity and other conditions, and sometimes requires maternal movement to change the fetal position for continued observation. Most of the fetal anomalies can be detected during this period, but there are still some anomalies that occur in late pregnancy, such as structural anomalies in the brain and heart anomalies. Ultrasound is only an observation of the general morphology of the organs and cannot identify whether the organs are functioning properly. Ultrasonography in late pregnancy. The general development of the fetus should be evaluated. The ultrasound report will describe what can be shown and what is not clear or less clear. The focus of this period is to provide clinical indications of fetal size, biophysical score, placental maturity and amniotic fluid volume. In addition to these several examinations, emergency ultrasound is also needed to observe some items such as cervical length and placenta-cervical relationship if discomfort occurs during pregnancy, and transvaginal or perineal ultrasound is needed if necessary, as transvaginal ultrasound is more accurate for diagnosis of placenta praevia, etc. However, due to the limitations of ultrasound technology, prenatal ultrasound cannot detect all abnormalities and cannot predict the later development of the fetus, so ultrasound diagnosis cannot be equivalent to clinical diagnosis.