Prenatal Screening for Down Syndrome
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p434-435
2025-11-29
172
Down syndrome is, in about 95% of cases, the result of non- disjunction of chromosome 21 in oogenesis or spermatogenesis (trisomy 21). The gold standard laboratory test to diagnose a fetus with Down syndrome is karyotype analysis by chorionic villus sampling or amniocentesis. The latter are invasive techniques associated with a low risk of fetal loss. A non-invasive alternative, and therefore not associated with any risk to the fetus, is the application of tests that estimate risk by combining information from ultrasound examination and/or biochemical examination of maternal blood with maternal age and gestational age. These tests provide the result in a number; if this value exceeds a specific cut-off, the test will be positive, and the woman may undergo more invasive investigations, such as amniocentesis and villocentesis. In particular, ultrasonography evaluates nuchal translucency, which is the measurement of the thickness of the subcutaneous tissue at the nape of the neck of the fetus, that is, the space between the skin and the spine. The greater the thick ness of nuchal translucency, the greater the probability that the fetus has Down’s syndrome. Biochemical examinations, on the other hand, are based on the evaluation of serum markers such as human chorionic gonadotropin (total or free beta fraction, hCG), pregnancy-associated plasma protein A (PAPP-A), alpha-fetoprotein (AFP), unconjugated estriol (uE3), and inhibin A. Table 1 shows the main tests available for assessing the risk of Down syndrome. It is important to note that a positive result from these tests does not indicate that the fetus will have Down syndrome.
Table1. Tests to assess Down syndrome risk
PAPP-A is a glycoprotein of placental origin, produced mainly in the syncytiotrophoblast.
AFP is a glycoprotein secreted first by the yolk sac and then by the fetus’s liver It has the same functions as albumin; immediately after birth, its concentrations progressively decrease until it is absent. AFP can cross the placenta and reach the maternal circulation, where its levels will be directly proportional to those in the fetal circulation.
Estriol is a steroid hormone produced in unconjugated form exclusively during pregnancy by the placenta and fetal organs: the fetal adrenal glands produce dehydroepiandrosterone sulfate (DHEA-S), which is hydroxylated by the fetal liver to 16-hydroxy-DHEA-S and subsequently metabolized by the placenta to form unconjugated estriol. The uE3 secreted by the placenta then passes into the maternal circulation; after conjugation in the liver, it is subsequently eliminated in the urine. The levels of uE3 increase in the maternal circulation as pregnancy progresses, especially in the third trimester; they are an index of the functionality of the fetoplacental unit.
Inhibin A is a dimeric glycoprotein produced by the corpus luteum and placenta during pregnancy. Generally, in the circulation of a mother of a fetus with Down’s syndrome, there should be, compared to a normal fetus, an increase in the levels of hCG and inhibin A and a decrease in the levels of PAPP-A, uE3, and AFP.
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