Down syndrome, also known as trisomy 21 or congenital stupidity, is caused by the presence of an extra third chromosome 21 in the genome and is characterized by mental retardation, delayed parenting, peculiar facial features and multiple small malformations. Due to the lack of effective treatment, prenatal screening and prenatal diagnosis to reduce the number of children born with the disease are currently the most recognized preventive measures in China and abroad. Serologic prenatal screening of pregnant women has the advantages of being non-invasive, rapid and economical, and can identify high-risk pregnant women with Down syndrome from the population being screened, so it is widely used to screen pregnant women of appropriate gestational age. At present, domestic institutions mostly use midtrimester serological screening, i.e., a comprehensive analysis using the triplet serological index AFP+uE3+hCG with age at 14-21 weeks of gestation to determine the risk of developing Down’s syndrome in babies. Recent studies in China and abroad have shown that PAPP-A and hCG levels can be measured in early pregnancy (7-13 weeks) in combination with the thickness of the posterior nuchal translucency (NT) on ultrasound as a “triplet” screening indicator for Down’s syndrome in early pregnancy. The results of foreign large sample studies show that the detection rate of Down’s syndrome by midtrimester duplex screening (AFP+hCG) alone is about 60%, the detection rate of Down’s syndrome by midtrimester triplet screening alone is about 70-74%, and the detection rate of Down’s syndrome by early pregnancy “triplet” screening alone is about 82-87%. The detection rate of Down’s syndrome can be increased to 94-96% with complete sequential screening in early and middle pregnancy, which can significantly reduce the risk of Down’s syndrome at birth.