Down’s syndrome is a prenatal screening test to check for Down’s syndrome in the fetus. Down syndrome, also known as trisomy 21, is a genetic disorder caused by chromosomal developmental abnormalities. It is a genetic disorder that causes mental retardation, growth retardation, special facial features and a series of other symptoms in the affected child. At present, there are two types of screening tests, one is an ultrasound and the other is a venous blood test. The main ultrasound test is the nt measurement, which is the measurement of the thickness of the nuchal translucency layer, and is usually performed between 11 and 13 weeks of pregnancy, and should be less than 3 mm under normal circumstances, and more than 3.5 mm is considered abnormal. Another method is to draw about 5 ml of maternal venous blood and the test is performed between 15 and 20 weeks of gestation. The mother’s blood is tested for alpha-fetoprotein, human chorionic gonadotropin, free estriol and inhibin a. This is combined with the mother’s age and some other conditions of the fetus to assess whether the fetus in the womb is likely to have Down’s syndrome. If the test result is high risk, it does not necessarily mean that the fetus has Down’s syndrome, because Down’s syndrome screening is only a screening method and not a confirmatory method, if it is high risk, then further amniocentesis or non-invasive DNA testing is needed to confirm the diagnosis.