B ultrasound NT refers to the thickness of fluid accumulation in the subcutaneous tissue behind the fetal neck, referred to as the nuchal translucency layer. its normal value should be 3mm when it can suggest the possibility of chromosomal abnormalities, but it cannot be an absolute diagnostic indicator, and can be further investigated by amniocentesis. when NT thickens, the pregnant woman refuses amniocentesis and should carefully consider the future health of the fetus and actively cooperate with other examinations by the doctor. If the result is abnormal after 13 weeks of pregnancy, it should be combined with cord blood puncture for chromosome screening. NT value after 20 weeks of pregnancy is basically no guidance.