Prenatal ultrasound diagnosis is one of the essential items for prenatal diagnosis. The Ministry of Health’s Regulations on Prenatal Diagnostic Technology stipulates six major lethal malformations that should be diagnosed at 18-24 weeks: anencephaly, severe brain bulge, severe open spina bifida, severe thoracoabdominal wall defect with visceral exostosis, single-chambered heart, and lethal chondrodysplasia. If the above abnormalities are detected by ultrasound, pregnant women are advised to go to a hospital with prenatal diagnosis to confirm the diagnosis. I. Anencephaly Anencephalic children are affected by genetic or environmental factors during the embryonic period, resulting in incomplete closure of the neural tube, forming anencephaly, spina bifida, etc. Environmental factors can act directly or indirectly on the embryo, especially the third and fourth weeks of embryonic development, which have the greatest impact on the nervous system. It can be detected by ultrasound as early as 11 weeks of gestation, and the diagnosis can be confirmed at 14-15 weeks of gestation. Ultrasound images show an incomplete or missing cranial halo, atrophy or absence of brain tissue, a normal lip and nose on the fetal face, protruding eyes, a very short neck, and mostly combined with excess amniotic fluid. Second, severe brain bulge Brain bulge is due to cranial defects, high pressure in the cerebrospinal cavity, intracranial structures herniated out of the skull through the defect. It occurs mostly in the midline of the occipital region, and most of the bulging material is cerebrospinal fluid, and severe patients will have bulging brain tissue. Ultrasound images: irregular fetal cranial halo, interrupted cranial continuation, and localized outward expansion of cystic masses. Open spina bifida is caused by the failure of the posterior neural foramen of the neural tube to close during embryonic development. The main feature is a spinal deformity caused by the failure of the two dorsal vertebral arches to fuse, with the spinal membrane or spinal cord exposed outward through the incompletely closed spinal column. The dorsal skin of the lesion is defective, and some or all of the components of the spinal canal expand backward through the spinal defect. The earlier the deformogenic factor, the higher and more extensive the site and the more complex and severe the degree of neurological deformity produced. Open spina bifida includes the following five types: 1. Spina bifida is a malformation that occurs before the 28th day of embryonic development. The spinal canal, spinal membrane and spinal cord are all split, with the thoracolumbar segment being the most common site. The original neural plate, neural groove and superior closed neural canal can be seen at the deformity, and there is often cerebrospinal fluid spillage, so there is a potential risk of meningitis and ventriculitis after birth. Neurological disorders are severe, and the lesion is often completely paralyzed below the segment. 2, spinal cord spinal membrane bulge for embryonic development after 28 days of malformation, more often seen in the lumbar or lumbosacral segment, often with sac formation. The spinal cord and nerve roots are either exposed or protrude into the bursa. The wall of the bursa may rupture, producing a cerebrospinal fluid fistula. Neurological deficits can be mild or severe, depending on the extent of the deformity. 3.Half of the spinal cord bulge is manifested as a slightly deviated spinal cord bulge from the midline, often only one side of the half of the spinal cord deformity, while the other half of the spinal cord is relatively intact. 4, hydrocephalic spinal cord spinal distension this deformity is rare, the contents of the distension contains an enlarged central canal and dysplastic spinal cord nerve tissue. There is often a more complete capsule, and spinal bulge is not easily distinguished. 5, spinal membrane bulge late embryonic malformations arising from the formation of spinal membrane bulge. The bulging bursa contains only cerebrospinal fluid. It is rare to produce cerebrospinal fluid leakage and meningitis. Ultrasound images show a strong echogenic band in the vertebral arch, a “V” or “U” shaped spinal canal splitting to the left or right, and a bulging sac of variable size at the site of spina bifida, which is soft and closely associated with the fetal body, or a “The spina bifida occurs at the site of elevated sacs of varying size, soft, and closely associated with the fetal body, or with signs such as “lemon head” and “banana brain. Severe open spina bifida can be screened by ultrasound at about 13 weeks of gestation. 4. Severe thoracoabdominal wall defect with visceral ectopia Severe thoracoabdominal wall defect with visceral ectopia is very rare. Fetal thoracic wall defect with full abdominal wall defect resulting in malformation of internal organs such as heart, liver, spleen and intestinal canal prolapse. Ventral fissure malformation occurs due to the natural regression of the right umbilical vein, which ruptures at the right side of the umbilical cord where the perineum is weak. It can usually be detected by ultrasound at 12 weeks of gestation. V. Single-chamber heart Single-chamber heart refers to the heart malformation single atrium single ventricle (two-chamber heart), which is a heart formation process that divides the heart into two halves with two mid-ridges: the atrial septum and ventricular septum are completely absent, so that the heart formation stays at the stage of two-chamber heart. Usually ultrasound can diagnose a single-chambered heart at 18-24 weeks. VI. Lethal chondrodysplasia Chondrodysplasia ultrasound shows extremely short limbs, curved limbs, anterior convexity of the spine, narrow chest cavity, cloverleaf head, and large head. For fetuses with a more definite gestational week, measurements of long bones (including femur, humerus, tibia, fibula, radius and ulna) that are more than 2 times the standard deviation below the predicted value should be considered abnormal, but if the fetal long bones are in the 3rd percentile while the growth interval is normal, it may be a small for gestational age weight baby due to maternal factors or other fetal genetic factors; when the gestational week is uncertain, refer to femur length/plantar length <0.87 , or the length of the bone is significantly shorter than the normal predicted value, then short limb deformity should be considered. When all long bone lengths of the extremities are < 4 times the standard deviation of the normal gestational week mean and the femur length/plantar circumference ratio is < 0.16, this is a severe short limb deformity and lethal skeletal developmental abnormalities should be alerted. Ultrasound can identify lethal chondrodysplasia from non-lethal syndromes by 24-28 weeks. Early diagnosis allows termination of the pregnancy before the fetus becomes viable and also avoids futile measures at delivery. Fetal chromosomal or related genetic tests should be performed to rule out chromosomal aberrations and genetic abnormalities, such as trisomy 21 and trisomy 18 children often present with short long bones, as lethal chondrodysplasia is autosomal dominant.