Some pregnant mothers have had an NT test or even had the experience of receiving multiple scans, i.e. after the first scan they were told to go for a walk, have something to eat, etc. Sometimes it takes three or four times to finish the test. And many pregnant women don’t even know what such a troublesome NT test actually does and what it can do, and there are those who believe that NT tests can screen for all. Here’s an introduction to this NT test!
I. What is NT
NT, the nuchal translucency layer, refers to the thickness of the fluid accumulation within the subcutaneous tissue fluid at the back of the fetus’ neck, which is reflected on the ultrasound sonogram as 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.
II. How NT is 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.
III. How to perform NT examination
Since NT is not a mandatory national program, it requires pregnant women to understand the need for NT screening and to take the initiative to undergo NT scan.
NT examination is applied to ultrasound scan and detected by 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.
IV. The best time for NT examination
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 45-84 mm. 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 based on the last menstrual period, not the gestational week calculated on the early ultrasound based on the fetal development.
V. Need for 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 layer, the higher the probability of fetal abnormalities.
It is necessary to do fetal nuchal translucency (NT) test as the first step to rule out fetal anomalies during pregnancy checkup.
VI. Clinical significance of NT test
1.NT measurement is an important part of ultrasound screening in early pregnancy
NT examination as a screening test is an effective imaging method for early detection of fetal abnormalities. The thicker the NT is, the higher the chance of fetal abnormality and the more serious the abnormality is.
2.Thickness can indirectly evaluate the possibility of fetal abnormality
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 when the false positive rate is 1%, the detection rate is 60%, indicating that NT screening is feasible , effective. 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 be seen 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 worth mentioning that a normal NT value does not mean that there is no problem, and the 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 the ultrasound examination is more reproducible and can better help the obstetrician to determine the gestational week. After 13 weeks + 6 days of gestation, it is no longer accurate to estimate the gestational week by the fetal head-rump diameter.
4.Some anomalies are detected earlier
During this period, if the fetus has large malformations, 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 and observe. Many international obstetrics centers have made the measurement of nuchal translucency layer thickness from 11 weeks to 14 weeks of gestation as one of the routine examination items.