The NT is an ultrasound measurement of the thickest subcutaneous anechoic hyaline layer of the fetus’s nuchal collar and is used to assess the risk of Down’s syndrome, which is an early screening test for Down’s syndrome. The best time for NT screening is between 11-13 weeks + 6 days of pregnancy, which can be measured by transabdominal or vaginal ultrasound, but transabdominal ultrasound is preferred because it is more convenient, no need to fast or hold urine before NT screening, and no need to be nervous during the test, which usually takes 10-20 minutes to complete. If NT > 3mm, it may be related to fetal chromosomal abnormalities and further chromosomal tests such as amniocentesis or non-invasive DNA are needed to rule out whether the fetus has chromosomal abnormalities.