Down syndrome, or trisomy 21, also known as congenital stupidity or Down syndrome, is a disease caused by an extra pair of chromosome 21. Chromosomes are the substances that contain human genetic information. There are 23 pairs of normal chromosomes, the first 22 pairs are called autosomes, and the 23rd pair is sex chromosomes, which are the key substances that determine gender, except for these are autosomes, which are the same for both men and women. For example, human beings are equivalent to a bus, including the driver can carry 46 people, the driver’s seat (equivalent to sex chromosomes) there are two, the other is the passenger seat (equivalent to autosomes), now there is an extra seat, because of the extra person’s eccentric character, bad habits, the carriage harmony is broken, quarrels and fights began to increase (that is, the extra chromosome is not good This is not a good thing). Down’s syndrome is one of the diseases that cannot be cured medically so far, so the measures to control the occurrence of Down’s syndrome mainly lie in preventing and reducing the number of children born with the disease. What does a child with Down syndrome look like? What are the signs of Down syndrome other than the often described mental retardation? When observing a child with Down syndrome, I first notice their face. Their eyes are wider than normal apart, their nose is low and flat, and their tongue is fat, often sticking out and drooling. When examining the palm of the hand, I would see a palm stripe running horizontally from the left side to the right side (medically known as a through palm and palmistry known as a broken palm). In addition, they have severe multiple congenital malformations, such as congenital heart disease, and are prone to various infections due to low immune function, and most children die before reaching adulthood. In order to prevent the birth of children with Down’s syndrome, early detection and treatment should be done, so prenatal screening for Down’s syndrome is very important. The screening for Down syndrome is divided into early screening, mid-pregnancy screening and combined early and mid-term screening. Early screening for Down’s syndrome focuses on pregnancy-associated protein A (PAPPA) and free chorionic gonadotropin (hCG) levels in the serum of pregnant women in early pregnancy and the thickness of the posterior nuchal translucency (NT) on ultrasound. The midterm Down syndrome screening test detects midtrimester serum levels of alpha-fetoprotein (AFP), free chorionic gonadotropin (hCG) and free estriol (uE3). To improve the detection rate of Down’s syndrome screening, the combination of pregnancy-associated protein A (PAPPA), free chorionic gonadotropin (hCG) and posterior nuchal translucency (NT) on ultrasound, as well as AFP, free chorionic gonadotropin (hCG) and free estriol (uE3) in maternal serum in mid-pregnancy is called combined early and mid-term screening. The risk of having a child with Down’s syndrome is calculated based on the increase or decrease of these markers in the serum of the pregnant woman, together with her due date, age, weight and week of pregnancy at the time of blood collection. When the results indicate a critical risk or high risk, you will need to consult your prenatal doctor. When should I be screened for Down’s syndrome? Either early or late screening can affect the accuracy of the results, so it is important to consult your doctor in advance to determine the timing of your screening. The best time for screening for Down’s syndrome in early pregnancy is between 9-13 weeks + 6 days, and the best time is between 11-12 weeks; the best time for screening for Down’s syndrome in mid pregnancy is between 14-20 weeks + 6 days, and the best time is between 16-18 weeks. If a pregnant woman misses the screening test for Down’s syndrome, the test cannot be performed and can only be performed by amniocentesis or non-invasive tests to analyze the fetal karyotype for diagnosis. Down’s syndrome screening tests can detect 60 to 70 percent of Down’s syndrome fetuses. However, it is important to clarify that the screening test can only help determine the chance of the fetus having Down’s syndrome, but it cannot definitively determine whether the fetus has Down’s syndrome. This is because it is also influenced by maternal age, weight, gestational week, fetal secretion of alpha-fetoprotein, placental secretion of human chorionic hormone, drug factors, genetic factors and other factors. When the screening result is low risk, there is no guarantee that the fetus will not develop the disease. Because screening for Down’s syndrome is only a probability test, a high-risk group only means that the fetus is more likely to be a child with Down’s syndrome, and a low-risk group may also be a child with Down’s syndrome. When the result of Down syndrome screening shows “high risk”, you should not be overly nervous as it only indicates that the fetus is more likely to have Down syndrome than the average pregnant woman. When the risk is high, the next step is to confirm that the baby is indeed a child with Down syndrome through prenatal diagnosis. The common technique used for prenatal diagnosis is amniocentesis, in which a needle is inserted into the amniotic fluid through the mother’s abdomen under ultrasound guidance, and the amniotic fluid is extracted for chromosomal analysis of the fetal cells to confirm the diagnosis. There is also a non-invasive technique in which some of the chromosomes are analyzed by drawing the blood of the pregnant woman.