What is Down’s syndrome screening? Down’s syndrome, also known as trisomy 21, was first described in 1866 by Dr. Down (full name is longer: John Langdon Haydon Down) in England, hence the name Down’s syndrome. Dr. Down did not know the cause of the disease until 1966, 100 years later, when it was discovered that Down’s syndrome was a congenital disorder caused by the fact that the child had three chromosomes, one more than normal, on chromosome 21. Children with Down syndrome are mentally retarded, unable to take care of themselves, and suffer from multisystem complications that are incurable due to the congenital defect, causing a heavy emotional and financial burden on the family. Therefore, there is a need for a method to detect children with Down’s syndrome as early as possible and terminate the pregnancy in time to reduce the number of children born with this condition. Before 1984, screening for Down’s syndrome was performed by amniocentesis in older women (>35 years old), and the chromosomes of amniotic fluid cells were examined directly, which was not easily accepted because it was an invasive test and could be harmful to the mother and fetus. Since 1984, it has been found that the concentration of some serum markers in the maternal blood of Down’s syndrome children differs from the concentration of the corresponding serum markers in normal mothers, so that the concentration of these markers can be measured and combined with other information to predict the probability of Down’s syndrome in the fetus. These serum markers include: plasma protein A (PAPP-A), chorionic gonadotropin (β-hCG), alpha-fetoprotein (AFP), free estriol (μE3), and inhibin A (Inhibin-A). (1) Screening can be performed in early pregnancy (10-13+6? weeks), mainly by duplex screening with the combination of two serum markers, PAPP-A and β-hCG, which is less commonly used in China. (2) It can also be performed in the middle of pregnancy (15-20+6? weeks), mainly the triple screening method with the combination of AFP, β-hCG and μE3 serum markers and the quadruple screening method with the combination of AFP, β-hCG, μE3 and Inhibin-A serum markers. The most widely used screening method in China is the triple screening method, and we will focus on this screening method today. Why are maternal serum markers abnormal in children with Down’s syndrome? AFP is mainly synthesized by the fetal liver cells and released into the maternal blood through passive transport. The AFP level in the blood of normal pregnant women will gradually increase with the development of the fetus, reaching a peak at 30 weeks of gestation. β-hCG is a glycoprotein secreted by placental trophoblast cells, and its concentration peaks at about 10 weeks of gestation and decreases after 1 to 2 weeks. The maternal β-hCG concentration in Down’s children is higher than that in normal mothers. The adrenal cortex of Down’s fetuses is poorly developed, resulting in reduced synthesis of dehydroepiandrosterone, which results in lower maternal μE3 concentrations in Down’s children than in normal mothers. As we mentioned earlier, maternal serum markers of Down’s syndrome differ from those of normal mothers, and in order to compare the two, it is necessary to know the concentration of the corresponding serum markers in normal mothers. Multiples of Median (MoM) is the value obtained by dividing the measured value of a serum marker in a pregnant woman by the median value of the corresponding gestational week in a normal pregnant woman. The calculated MoM value is also corrected by dividing the calculated MoM value by a correction factor because the concentration of the serum marker is also influenced by factors such as maternal race, weight, presence of diabetes, smoking, and singleton, twin, or multiple fetuses. The correction is usually done by measuring the serum marker concentrations of a sufficient number of pregnant women with the appropriate risk factor at each week of pregnancy to calculate the corresponding risk factor. Calculation of risk level Total risk level = Age-specific risk level × LR(AFP) × LR(β-hCG) × LR(μE3) Calculation steps: 1. is the risk calculation factor, P = 0.000627+e[-16.2395+(0.286×age)] 2. The likelihood ratio LR is the ratio of the probability of arriving at a particular screening test among those with the disease to the probability of arriving at this probability among those without the disease. The calculation of the likelihood ratio (LR) requires a complex formula to calculate the MoM mentioned earlier and then bring the MoM into the statistical analysis to obtain the LR for each serum marker separately. The LR for each serum marker is a composite indicator often used in screening tests and can reflect both the sensitivity and specificity of the screening test. How to interpret the results of Down screening? The results of Down screening are expressed as a risk level, which indicates the risk of a pregnant woman’s fetus having Down’s syndrome, and is not a confirmatory result. For example, a risk level of 1:800 for a pregnant woman with Down’s syndrome means that one child with Down’s syndrome will be born out of 800 pregnant women with the same condition. It is important to note that a high risk level does not necessarily mean that a child with Down’s syndrome will be born, but only that the probability is high; similarly, a low risk level does not necessarily mean that a child with Down’s syndrome will not be born, but only that the probability is low. The process of handling high-risk results Usually, a risk level of 1:270 or higher is considered high-risk (the cutoff value may vary depending on the methodology). If the gestational week does not match, the risk level should be recalculated according to the gestational week of the B ultrasound, and further verification of maternal information including age, weight, race, presence of diabetes, and whether or not she smokes should be done. If the information matches the filled gestational week and is accurate, further amniocentesis for cytological chromosome examination or non-invasive chromosome examination will be done. A few frequently asked questions 1. How to calculate the gestational week The gestational week is very important in Down screening because the indicators in all three sera change dynamically with the gestational week and the median value of a normal pregnancy corresponds to the corresponding gestational week, and an error in the gestational week may lead to an incorrect risk level. The gestational week is usually calculated in two ways: firstly, by the time of the last menstrual period; secondly, by measuring the fetal biparietal diameter by B-ultrasound. If the difference between the two methods is more than 10 days, the B-ultrasound gestational week prevails, and if the difference is less than 10 days, both are acceptable. As mentioned above, age, weight, race, diabetes, smoking, etc. can affect the results. In clinical practice, we often encounter some pregnant women who do not pay enough attention to these information when filling out the Down screening checklist, thinking that it is almost enough, some weight is estimated approximately, some pregnant women fill in their age according to Chinese tradition, etc., which can affect the accuracy of screening. These can have an impact on the accuracy of screening.