The nt test is actually an ultrasound test that measures the maximum thickness of the soft tissue between the skin and fascia layers on the back of the fetal neck, reflecting the accumulation of lymphatic fluid in the subcutaneous tissue. nt also means cervical hyaline layer. The fetal lymphatic system is not well developed before the 14th week of pregnancy, and a small amount of lymphatic fluid collects in the cervical lymphatic sacs or lymphatic vessels to form the nt. The nt test is the cervical translucency test, which focuses on the fetal cervical examination and the thickness of development, and is directly related to the development of the human body and body morphology. It is also one of the diagnostic bases for early Down’s syndrome screening, the purpose of which is to diagnose chromosomal disorders in the early stages of pregnancy and to detect multiple causes of fetal abnormalities. nt measurement is performed under ultrasound, and a thickness of about 2.5 mm is considered abnormal. nt is too thick to indicate the possibility of chromosomal abnormalities in the fetus, especially the increased risk of Down’s syndrome. Of course, if there is a thickening of nt, it does not mean that all fetuses must have problems. Further tests such as chorionic villus aspiration or amniocentesis are needed to determine this. Usually the nt test is performed between 11 and 14 weeks of pregnancy. Before 11 weeks the fetus is small and it is difficult to observe the posterior nuchal translucency, while after 14 weeks the fetus gradually develops and may absorb the excess fluid from the nuchal translucency, affecting the test results.