Early screening and middle screening during pregnancy mainly check the possibility of chromosomal abnormalities in the fetus. 1. Early screening: generally refers to NT examination, usually conducted between 11-14 weeks of pregnancy, mainly through ultrasound examination of fetal nuchal translucency thickness to determine whether there is a possibility of chromosomal abnormalities, the normal value is generally less than 2.5mm, when the result is more than 3mm, it is recommended that non-invasive DNA or amniocentesis and other tests in a timely manner to further rule out the possibility of chromosomal abnormalities. 2. Middle screening: usually refers to the Down’s syndrome examination, the examination time is between 15-20 weeks, mainly through the drawing of high-risk pregnancy to take the pregnant woman’s venous blood to determine whether there is a possibility of Down’s syndrome, but also part of the pregnant woman can choose non-invasive DNA examination. Early and mid-term screening are the main ways to screen for the risk of chromosomal abnormalities, and it is recommended that pregnant women undergo regular checkups.