1.What is NT?
In this article, we talk about NT not to discuss New Technoly, New Taiwan Dollar, or the sensitive National Treatment, but the Nuchal Translucency, which is the accumulation of fluid in the subcutaneous tissues at the back of the fetal neck. The thickness of NT is also the maximum thickness of the soft tissue between the skin layer and the fascial layer at the back of the fetal neck.
2. How is NT formed?
Before the normal embryonic lymphatic system is complete, a small amount of lymphatic fluid collects in the cervical lymphatic sacs or lymphatic vessels, forming the nuchal translucency layer. After 14 weeks, when the lymphatic system is well developed, the accumulated lymphatic fluid rapidly drains to the internal jugular vein, and the nuchal translucency layer then disappears rapidly. Whether due to genetics, anatomical structure or infection, the delayed communication between the lymphatic vessels and the jugular vein may cause obstruction of lymphatic return, resulting in the accumulation of excessive lymphatic fluid in the jugular region, which leads to the thickening of the hyaline layer and even the development of cystic lymphadenoma (lymphatic hydrocystic tumor) by mid-pregnancy.
3.What is the significance of NT?
More often than not, it is the obstetrician who asks us to measure the NT thickness by ultrasound, the specific significance of which is long and complicated. early stage of cystic lymphadenoma, etc. However, it should be noted that about 80% to 90% of NT abnormalities in fetuses are transient lesions with normal final results.
If a thickened hyaline layer is found, in addition to the recommended fetal karyotype check, close follow-up is needed for those with normal karyotype to rule out congenital heart development abnormalities.
4.How is NT measured?
This part is the focus of our ultrasound attention. The following text and pictures in bold are from the Chinese textbook of FMF Foundation.
(1) It should be performed at 11-13+6 weeks, when the fetal head and rump length is 45-84 mm.
If your obstetrician asks you to measure the NT of a fetus at 20 weeks of gestation, you can rightly refuse and despise it.
(2) A median sagittal view of the fetus should be obtained and the NT should be measured in the natural position.
If the fetus is too nervous, you can use “fetal language” to reassure him/her that the ultrasound is harmless and that he/she is relaxed.
(3) Always zoom in on the ultrasound image so that only the head and upper chest are shown.
The fetus is the main character, and you should let him or her occupy at least 3/4 of the whole screen, which you can refer to the classic image of “The Legend of Wu Tai Tou”.
(4) It should be measured in the transparent zone with the widest distance between the skin and the soft tissue on the cervical spine.
The meaning of this statement is to remind you not to take the bull by the horns, the open zone is the best target for you to chase.
(5) Be careful to distinguish between fetal skin and amniotic membrane.
Sometimes what you think is simple is often easier to miss. Pat the PP of the fetus, let him move a little, you may be able to identify.
(6) The transverse vernier scale should not be placed on the cervical fluid, but on the border of the white line until the two are fused and the transverse scale is not easily visible.
As the “judge” scale, should have Chinese characteristics, never interfere with the internal affairs of other countries.
(7) In scanning, the number of measurements should be more than once, and record the maximum value obtained from the measurement.
Don’t be neutral, don’t be even, and demand the maximum benefit.
(8) When the fetus’ neck is surrounded by the umbilical cord, its NT thickness should be measured above and below the umbilical cord, taking the average of the two values.
The fetus is actually very well behaved, you wait for him for half an hour or he will not be in a mood.
(9) The ultrasound instrument used must have a high resolution, image playback function, and a light scale that can provide accurate measurement down to 0.1 mm.
5. What are the criteria for determining NT?
Normally, NT ≥ 2.5 mm is considered abnormal at 10-14 weeks of gestation. However, the NT abnormality should be judged not only by the gestational week but also by the age of the pregnant woman, and the range of abnormal values should be relaxed if the pregnant woman is of advanced age (above 35 years old).