Pregnant women undergo a test called Down’s syndrome screening at 15-20 weeks of gestation, which initially screens for pregnant women at high risk for neural tube defects (such as anencephaly, spina bifida, etc.), 13-3 syndrome, 18-3 syndrome, 21-3 syndrome. （The results are divided into high-risk (risk) and low-risk (risk) groups, and pregnant women in the high-risk group are diagnosed through amniocentesis to obtain fetal cells for karyotype analysis. Mothers-to-be often ask what to do if they are at high risk, should they have amniocentesis? Will the fetus be harmed? What is the accuracy rate? So it is important to understand a few things first. Down’s syndrome screening is generally done by using the results of maternal serum AFP, beta chorionic gonadotropin (beta-HCG) concentration, free estriol, combined with maternal age, weight, maternal history and gestational weeks to calculate the risk factor for the corresponding disease, the result is a risk value, such as 1:2000, which means there is a 1 in 2000 chance of having the disease. The parameters that enter into the analysis vary slightly depending on the software system used, and some software also involve fetal cervical glial thickness in the calculation, so that the reference risk coefficient varies from unit to unit. It should be noted, however, that whatever the result, which is a screening result, there is a certain amount of error, i.e. false negatives and false positives, regardless of the high and low risk groups. A false positive is when the report shows an abnormality while the fetus is normal, and a false negative is when the report is normal and the fetus is a sick child. Whether to perform amniocentesis or not depends on the specific problem. In principle, after ruling out the confounding factors (usually the gestational week error), it is recommended to do a confirmatory diagnosis, i.e. amniotic fluid fetal cell chromosome analysis. If the risk is low, continue the pregnancy test. The gestational week error is mainly used for some pregnant women with irregular menstrual cycles or long cycles (more than 37 days), and the analysis is performed after correcting the gestational week to exclude misdiagnosis due to the gestational week error. DOWn’s syndrome (DS) is the most common chromosomal disorder with a prevalence of 1.5 per 1,000 live births and is characterized by mental retardation, multiple congenital malformations and growth retardation. The birth of a child with DS brings a great burden to the family and society. If the diagnosis is made prenatally, the burden on the family and society can be eliminated. In foreign countries, amniocentesis is routinely done in advanced mothers over 35 years of age, although there is a slight risk of puncture, such as: causing infection, bleeding, miscarriage, etc., but the incidence is extremely low, and on balance, high-risk mothers-to-be should still have amniocentesis to confirm the diagnosis.