The NT test, also known as posterior nuchal translucency scan, is an ultrasound test that measures the thickest part of the subcutaneous anechoic hyaline layer in the nuchal region of the fetus and is used to assess whether the fetus has Down syndrome. This is the stage when the NT test is performed. Before the normal fetal lymphatic system is established, a small amount of lymphatic fluid collects in the cervical lymphatic vessels; after 14 weeks, the fetal lymphatic system gradually develops, and the collected lymphatic fluid rapidly drains into the internal jugular vein, and the test is more accurate at this time. In general, the normal range of posterior nuchal translucency is within 3mm. the thicker the nuchal translucency layer, the greater the probability of fetal abnormality. NT test measurements greater than 3mm are mostly abnormal, but due to individual differences, some normal fetuses also have more body fluids, which can easily affect the measurement results. If the posterior nuchal translucency is thicker, it does not necessarily mean that there is something wrong with the fetus. Pregnant women can consult their doctors to decide whether they need to undergo a definite diagnosis such as non-invasive DNA test or amniocentesis.