I. What is NT? NT stands for nuchal translucency. Assessment of nuchal translucency is the most reliable and widely used ultrasound indicator for trisomy 21. Cervical translucency is the transparent cervical space behind the fetus’s neck observed in the mid-sagittal plane during the ultrasound examination. When is NT done? In China, it is usually done between 11-14 weeks of pregnancy and the fetal head and rump length is between 45-84mm. How much NT is considered abnormal? In domestic studies, NT2.5mm is considered as the 95% threshold, and more than 2.5mm is considered as thickening. Fourth, why is NT thickened? 1.Aneuploidy: The most common aneuploidy associated with cervical hyaline thickening is 21-trisomy. However, the incidence of trisomy 13 (Patau syndrome), trisomy 18 (Edward syndrome), haplogroup X (Turner syndrome) and triploidy is also increased in fetuses with thickened nuchal translucency. The likelihood of aneuploidy increases as the thickness of the nuchal translucency increases to greater than the 95th percentile. For example, one study found that the incidence of aneuploidy was 7%, 20%, 50%, and 75% when the nuchal translucency thickness was between the 95th percentile (approximately 3 mm) and 3.4 mm, 3.5-4.4 mm, 5.5-6.4 mm, and greater than or equal to 8.5 mm, respectively. In this study, the majority of trisomy 21 fetuses had a nuchal translucency thickness less than 4.5 mm, and the majority of trisomy 13, trisomy 18 and Turner syndrome fetuses had a nuchal translucency thickness greater than or equal to 4.5 mm. 2. Structural abnormalities: The overall incidence of structural abnormalities in haploid fetuses with thickened nuchal translucency was 4-10%. (1) Congenital heart disease: Among the anomalies associated with cervical hyaline layer thickening, the most common one is heart defect, and the most common one among heart defects is septal defect. The overall incidence of congenital heart disease increases with increasing cervical hyaline layer thickness. Depending on the specific threshold used (95th percentile vs. 99th percentile), the overall risk of heart defects in whole-ploidy fetuses with thickened nuchal translucency is 2-6%, which is several times the baseline risk of moderate and severe congenital heart disease in fetuses in the general obstetric population (0.6%). (2) Non-cardiac anomalies: For infants with fetal nuchal translucency thickness values greater than or equal to the 95th percentile, the risk of the following anomalies is approximately three times greater than for infants with nuchal translucency thickness less than the 95th percentile: hydrocephalus, pulmonary failure, hypoplasia and dysplasia, small bowel atresia and stenosis, osteodystrophy, and diaphragmatic anomalies. 3. Developmental and genetic syndromes: More than 100 developmental and genetic syndromes have been found to be associated with thickening of the nuchal translucency layer. The following disorders have been diagnosed in children with intrauterine cervical hyaline thickening: Noonan syndrome, multiple pterygium syndrome, Roberts syndrome, CorneliadeLange syndrome, congenital adrenocortical hyperplasia, spinal muscular atrophy, DiGeorge syndrome, Smith-Lemli-Opitz syndrome and multiple skeletal dysplasias. Some of these syndromes are associated with genetic mutations or structural abnormalities that can be detected prenatally, while others cannot be detected prenatally. 4. Twin transfusion: In monochorionic twins, a thickening of the nuchal translucency or a difference of greater than 20% in twin measurements can predict twin transfusion syndrome (TTTS). One study found that the risk of early fetal death or development of severe TTTS was greater than 30% when the difference in twin measurements was greater than 20%, whereas the risk was less than 10% when the difference was less than 20%. V. Natural course In normal fetuses, the thickened nuchal translucency usually resolves spontaneously by midterm pregnancy. If the thickening persists, it is indicative of an underlying abnormality. Persistent or massive nuchal translucency seems to indicate a poor prognosis even in an uncomplicated haploid fetus.