The normal range of ultrasound NT is less than or equal to 2.5mm. The Chinese word for NT is nuchal translucency, and nuchal translucency refers to the thickness of the fluid accumulation in the subcutaneous tissue at the back of the neck of the fetus, which is reflected in the sonogram as an echo-less layer in the subcutaneous tissue at the back of the neck of the fetus. The significance of measuring the NT thickness is that if the NT thickness increases, the likelihood of fetal abnormalities, i.e., chromosomal abnormalities, also increases. If the NT measurement is combined with maternal serum HCG, i.e. early Down’s syndrome, the detection rate can be further increased and false positives reduced. The gestational week for NT measurement should be strictly limited to 11-13 gestational weeks + 6, and the measurement should be made perpendicular to the skin, with the inner edge of the vernier placed at the outer edge of the echogenic N.