Down’s syndrome refers to chromosomal abnormalities such as trisomy 21, trisomy 18 and trisomy 13, which usually manifest as a variety of malformations and congenital mental retardation after birth, bringing a heavy burden to families and society. Early screening has a high detection rate of over 85%, including ultrasound NT and blood test, which is done between 11-13+6 weeks; mid screening has a 70% detection rate, which is usually done between 16-20 weeks. Usually you don’t need to do the mid-term screening if you have done the early screening, but if you can’t do the early screening, you can do the mid-term screening.