The NT test is best done between 11-13 weeks + 6 days of pregnancy. NT is a measurement of the fluid portion of the band between the skin and soft tissue on the back of the neck of the fetus, and is basically measured at 11-13 weeks + 6 days of gestation.NT is part of early Down’s screening, which also includes a blood draw for biochemical markers, and Down’s screening for trisomy 21 can be around 90%.NT can also help determine gestational age, and find out whether the fetus has any extremely severe structural malformations, such as anencephaly, extra-thoracic heart, missing limbs, etc., as well as the possibility of stillbirth. The normal value of NT is usually defined within 2.5mm in China and within 3mm abroad, but a thickened NT does not necessarily mean that the fetus has an abnormality, and most, especially those with mild thickening, can be normal fetuses. All prenatal screening results are not a single reading, need to be read through a variety of screening methods, such as early Tang, Tang, ultrasound malformation screening and non-invasive DNA together with joint screening, specific prenatal screening can consult the local hospital obstetrics and gynecology department.