Ultrasound analysis and thinking methods Ultrasound diagnosis of fetal central nervous system malformations thinking and analysis methods: ①associative analysis method. There are 3 important transverse sections of intracranial structures by ultrasound examination: horizontal transverse section of thalamus, horizontal transverse section of lateral ventricle and trans-cerebellar transverse section. Through systematic observation of these 3 transverse sections, about 95% of obvious malformations of central nervous system can be detected, but the abnormalities of intracerebral structures are often not single but interconnected. ②The principles of ultrasound diagnosis of neurological malformations. Emphasize that the disappearance and enlargement of certain structures is associated with certain malformations – often the initial clue to these malformations. Structural malformations in the brain are often multiple malformations rather than single malformations. To diagnose meningoencephalomalacia, it is important to have a cranial deficit and to note that brain parenchymal echoes are differentiated from small cystic lymphomas in the neck. Meckel-Gruber syndrome should be considered if there is meningeal brain expansion with polycystic kidney and polydactyly. (iii) Genetic ultrasound soft indicators related to the nervous system: Genetic ultrasound signs related to the nervous system include choroid plexus cyst, enlarged posterior cranial fossa pool, cerebellar deviation, cerebellar earthworm deletion (both complete and partial), mild dilatation of the lateral ventricles, and agenesis of the corpus callosum. If a single soft indicator is found to be positive, all soft indicators should be examined, and the more one positive means the greater the chance of having chromosomal abnormalities. If ultrasound reveals cerebellar earthworm deletion, enlarged posterior cranial fossa, strawberry-shaped head, combined overlapping fingers, small mandibles, etc., trisomy 18 chromosomal disorders are mostly considered; if whole forebrain, combined bilateral cleft lip and palate, multiple fingers, etc. are found, trisomy 13 disorders are mostly considered. The incidence of choroid plexus cysts is 1%, and 44% to 50% of fetuses with trisomy 18 have prenatal detection of choroid plexus cysts. Regular follow-up is required and chromosomal examination is recommended if the pregnant woman is over 35 years of age. Limitations of ultrasound diagnosis: As a safe and easy examination method, ultrasound has long been the main means of prenatal diagnosis of CNS malformations, but it also has certain limitations, its diagnosis is based on fetal morphological changes, and the detection rate is high if the morphological changes are large, but not easy to detect if the morphological changes are small. For example, continuous follow-up of brain or meningeal bulge disappears in a period of time and reappears after a period of time. Therefore, when screening fetus for malformations, we should master the timing of screening and examination indications, the differentiation of organs has been completed after 12 weeks of gestation, so the first ultrasound examination can be performed from 12 to 14 weeks of gestation to exclude anencephaly and other early large malformations; the second detailed and systematic screening for fetal malformations from 18 to 24 weeks is the most ideal, the fetal anatomical structure has been formed and can be shown by ultrasound during this period. The fetal size and amniotic fluid are moderate, less influenced by bone echo, and the image is clear, so most anomalies can be excluded during this period.