Down screening refers to the Down’s syndrome screening test, which is generally best performed at 12-14 weeks of gestation. In recent years non-invasive screening for chromosomal aneuploidy has been commonly used because of its high accuracy and wide range of gestational weeks. It is mainly used to determine the presence of chromosomal aneuploidy changes in the fetus by sequencing free DNA from maternal peripheral blood samples to screen for trisomy 21, trisomy 18 or trisomy 13. Prenatal diagnosis may be recommended when a pregnant woman has high-risk factors (e.g., age ≥35 years, couple with congenital or hereditary disease, etc.).