The NT test, also known as posterior nuchal translucency scan, is performed by ultrasound to measure the thickest subcutaneous anechoic hyaline layer in the nuchal region of the fetus to assess the possibility of Down’s syndrome, and is usually performed when the fetus is 45-85 cm long at head and hip. The fetus is of moderate size, with good mobility, and the nasal bone and skin structure of the neck are clearly visible. Before 1l weeks of pregnancy, the fetus is too small, the measurement is technically difficult and there is uncertainty in the test results; after 14 weeks of pregnancy, the system of fetal lymph starts to develop, lymph starts to circulate, the posterior nuchal translucency will gradually disappear and may not be measurable. Before the NT test, pregnant women can eat breakfast and do not need to hold urine, but they should maintain a good mood and interact with the fetus more. pregnant women can go out and walk around for a while during the NT test before continuing the test. The doctor may apply moderate pressure on the pregnant woman’s stomach to wake up the sleeping fetus and guide it to turn over to improve the efficiency of the examination. The whole NT examination takes about 10-20 minutes, and may be shorter if the baby cooperates well.