Mid-term Down screening is done at 15-20⁺⁶ weeks of pregnancy. Intermediate Down screening mainly refers to a woman’s mid-pregnancy screening for fetal chromosomal abnormality disorders, which is mainly performed by drawing blood from a vein, and is a more important fetal screening test. Midterm Down’s syndrome screening is best performed before 18 weeks, and if the test results show a gray zone or high risk, it is necessary to continue with non-invasive DNA or amniocentesis. Non-invasive DNA as well as amniocentesis tests take some time. If it is done later than 18 weeks, it may affect the non-invasive DNA as well as the amniocentesis. Secondly if there is an abnormality during the test, there is still time for follow up and examination.