NT test, also called posterior nuchal translucency scan, measures the thickest part of the fetal nuchal translucency in the subcutaneous area of the nuchal region by ultrasound, and is used to evaluate whether the fetus has Down’s syndrome or not. NT test is the first step to rule out the developmental abnormality of the fetus during pregnancy checkups, and the result is usually available on the same day or the next day, depending on different hospitals.NT test is best performed at the stage of pregnancy of (11-13) weeks + 6 days. This stage is performed. Before the normal fetal lymphatic system is established, a small amount of lymphatic fluid collects in the lymphatic vessels of the neck. 14 weeks later, the fetal lymphatic system gradually develops and the collected lymphatic fluid rapidly drains into the internal jugular vein, which makes the test more accurate. In general, the normal range of the posterior nuchal translucency is within 3mm. the thicker the nuchal translucency, the higher the probability of fetal abnormality. measurements larger than 3mm in NT examination 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 doctor’s opinion to decide whether they need to undergo non-invasive DNA testing or amniocentesis to make a clear diagnosis.