SD or Down syndrome is the most common chromosomal disorder and most of the children born with SD have extra chromosome ( number 21) in all of their cells, so the syndrome is also called trisomy 21. The cause of the disorder is a nonseparation of the same pair of chromosomes in mother's gametes. The only proven cause of the nonseparation is mother's age and it is proven that SD occurs far more often in mothers aged above 35 years. Therefore we can see that the incidence of aneuploidy rises with mother's age and is caused by the nonseparation of the chromosomes. SD causes a wrong transmission of the chromosomes during meiosis which results in one cell having a part or an entire extra chromosome. It mainly occurs in egg cell before fertilization, but can also occur in sperm cell in less cases. At the point, cells do not contain 46 but 47 chromosomes. Children with SD have characteristic appearance: slanted eyes, indentation of the fourth finger, sunken nasal root, twisted nose. Typical syndromes are also: mild to moderate intellectual disability, delayed speech development, shortened life expectancy as a consequence of heart defects. In order to detect possible chromosomal disorders on time, pregnant women are advised to take the early recombinant screening test during the eleventh and fourteenth week of pregnancy. The early recombinant screening test is the most efficient method for detecting 90 % pregnancies with SD and other chromosomal disorders. Unlike early screening tests which are done in the first trimester, 'double' and 'triple' tests are done in the second trimester. Those tests are determined from mother's blood and are less reliable, around 50-70%. Prenatal diagnosis of the trisomy 21 also uses some other invasive and noninvasive methods. The invasive ones include amniocentesis and CVS. Prenatal diagnosis nowadays also uses molecular methods such as flourescent in situ hybridization (FISH) and quantitative polymerase chain reaction ( Q-PCR) which are fast and have great sensitivity and specificity. Prenatal risk determinatation for SD development is based upon voluntary parental consent. Parents are introduced with the possible noninvasive and invasive methods of the sole test and diagnosis, and they should give their written consent. Since 2012, in The Croatian Register of Persons with Disabilities 1.446 people have been diagnosed with SD. The SD frequency is equal between both sexes. The largest number of registered persons, a staggering 58,2% belong to a younger age group (0-19 years). 7. LITERATURA