Some pregnant mothers have had an NT test, or even had the experience of receiving multiple scans, i.e. being told to take a walk, eat something, etc. after the first scan. Sometimes it takes three or four times to finish the test. Many pregnant women do not even know what such a troublesome NT test actually does and what it can do, and some people think that NT test can screen out all abnormalities of the fetus and exaggerate its usefulness. Here’s an introduction to this NT! 1.What is NT? NT is the nuchal translucency layer, which refers to the thickness of the fluid accumulation in the subcutaneous tissue fluid at the back of the fetus’ neck, reflected in the ultrasound sonogram, that is, the maximum thickness of the echogenic layer between the skin at the back of the neck and the subcutaneous soft tissue measured in the horizontal sagittal section of the fetal cervical spine. 2.How is NT formed? Before the normal fetal lymphatic system is established, a small amount of lymphatic fluid collects in the cervical lymphatic vessels, forming NT. after 14 weeks, the fetal lymphatic system is well developed and the collected lymphatic fluid rapidly drains into the internal jugular vein, and the NT, the hyaline layer, usually subsides. When lymphatic return is impaired, excessive lymphatic fluid collects in the cervical collar, thickening the translucent tissue between the skin and the connective tissue below it, and sometimes the lymphatic vessels dilate to form cervical edema or cervical hydrocystic tumor. 3.How to perform NT test? Since NT is not a national mandatory program, it is necessary for pregnant women to understand the necessity of NT screening and to take the initiative to undergo NT scanning, which is done by ultrasound detection using ultrasound. Therefore it is done at the ultrasound department of the hospital. As long as the conditions for the test are met, NT thickness detection can be performed. 4.The best time for NT test? The variation of NT is closely related to the week of pregnancy and the time to measure its thickness is strictly stipulated. It is performed during the time period of 11-13 weeks + 6 days of pregnancy, when the head-rump length is equivalent to 45-84mm. 11-13 weeks + 98% to 100% of the NT thickness can be measured, while 14 weeks drops to 11%. It is important to keep track of the timing of the test to avoid pregnant women not planning or missing an NT test. Note: The gestational week mentioned here is the gestational week calculated from the last menstrual period, not the gestational week calculated from the fetal development on the early ultrasound. 5.What is the need of NT test? Except for large structural malformations that appear in early stages. It is an ultrasound screening program in early pregnancy and can also be called a small screening for malformations. The thicker the nuchal translucency, the higher the probability of fetal anomalies. It is necessary to do fetal nuchal translucency (NT) test as the first step to rule out fetal abnormalities during pregnancy checkup. 6. Misconceptions about NT test: Some people think that NT test can detect all abnormal fetuses, answer: no. NT test is a screening and an effective imaging method for early detection of fetal abnormalities. The more significant the NT thickening, the higher the chance of fetal abnormalities and the more serious the abnormalities. a normal NT value does not mean that there is no problem. It is important to note that about 80% – 90% of NT abnormalities are transient, i.e. if the fetus does not have any chromosomal or cardiac problems, even if the NT is thickened, it does not mean that there is any problem. NT thickening simply indicates the need for the next step in the screening process. When NT is abnormal in early pregnancy, further tests should be performed, such as performing relevant chromosomal tests, etc. For those with normal karyotype, close follow-up is also needed to rule out congenital heart developmental abnormalities and other structural abnormalities. For those with normal karyotype, close follow-up is needed to rule out congenital heart development and other structural anomalies. Specific tests are required at the prenatal diagnosis clinic. It has been reported in various papers that 90% of fetuses with NT thickness >3mm are normal and 10% are abnormal; >6mm, 90% are abnormal. NT screening is like our current security screening, and NT thickening is like a suspicious person found during the screening process, which needs to be further investigated. So NT is only a marker for indirect diagnosis of fetal abnormalities. 7. Clinical significance of NT: ①NT measurement is an important element of ultrasound screening during early pregnancy. NT examination as a screening test is an effective imaging method for early detection of fetal abnormalities. The more significant NT thickening is, the higher the chance of fetal abnormality and the more serious the abnormality is. NT thickening indicates the need for the next examination. ②Thickness can indirectly evaluate the possibility of fetal abnormalities. When NT exceeds a certain thickness, the chance of fetal abnormality is higher. Mainly related to fetal chromosomal abnormalities, NT thickening can better evaluate the risk of fetus developing trisomy 21, trisomy 18, trisomy 13, etc. If the false positive rate is 5%, more than 75% of fetuses with Down syndrome and other serious chromosomal abnormalities can be screened by fetal NT, and the detection rate is 60% when the false positive rate is 1%, indicating that NT screening is feasible This indicates that NT screening is feasible, effective and efficient. It is also associated with structural malformations of the fetal congenital heart and is the most common cause of non-chromosomal abnormal NT thickening. As well as being associated with other structural malformations of the fetus. NT thickening can also occur in malformations of the skeletal system, diaphragmatic hernia, anterior abdominal wall defects (umbilical bulge), and fetal dyskinesia syndrome. It is also associated with spontaneous abortion. It is important to note that a normal NT value does not mean that there is no problem. NT test is only a marker for indirect diagnosis of fetal abnormalities. ③ Determining the gestational week. The measurement of fetal biparietal diameter and head-rump length during ultrasonography is more reproducible and can better help the obstetrician to determine the gestational week. Estimation of gestational week by fetal head-rump diameter after 13 weeks + 6 days of gestation is also no longer accurate. ④ Some anomalies are detected earlier. If there are any major abnormalities in the fetus, they will be detected earlier. ⑤ Easy to follow up and observe. Current research shows that prenatal detection by ultrasound is non-invasive, safe, convenient, accurate and easy to follow up. Many international obstetrical centers have made the measurement of nuchal translucency thickness from 11 weeks to 14 weeks of gestation a routine test.