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The extra chromosome content can arise through several different ways. The most common cause (about 92–95% of cases) is a complete extra copy of chromosome 21, resulting in trisomy 21. [92] [97] In 1–2.5% of cases, some of the cells in the body are normal and others have trisomy 21, known as mosaic Down syndrome.
Noninvasive prenatal testing (NIPT) is a method used to determine the risk for the fetus being born with certain chromosomal abnormalities, such as trisomy 21, trisomy 18 and trisomy 13. [1] [2] [3] This testing analyzes small DNA fragments that circulate in the blood of a pregnant woman. [4]
The most common abnormality the test can screen is trisomy 21 (Down syndrome).In addition to Down syndrome, the triple and quadruple screens assess risk for fetal trisomy 18 also known as Edwards syndrome, open neural tube defects, and may also detect an increased risk of Turner syndrome, triploidy, trisomy 16 mosaicism, fetal death, Smith–Lemli–Opitz syndrome, and steroid sulfatase ...
Trisomy 21. Fetal trisomy of chromosome 21 is the cause of Down's syndrome. This trisomy can be detected by analysis of cffDNA from maternal blood by massively parallel shotgun sequencing (MPSS). [61] Another technique is digital analysis of selected regions (DANSR). [61] Such tests show a sensitivity of about 99% and a specificity of more than ...
The trisomy 21 karyotype figure shows the chromosomal arrangement, with the prominent extra chromosome 21. Trisomy 21 is the cause of approximately 95% of observed Down syndrome, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete. [4]
The common autosomal trisomies (21, 18, 13) made up a smaller number of cases of mosaicism detected on CVS, but were more often confirmed in fetal tissue (19%). [3] On the other hand, the uncommon autosomal trisomies accounted for a greater number of placental mosaicism cases, but were less often confirmed in fetal tissue (3.2%). [ 3 ]
The presence of an extra chromosome 21, which is found in Down syndrome, is called trisomy 21. Trisomy 18 and Trisomy 13, known as Edwards syndrome and Patau syndrome, respectively, are the two other autosomal trisomies recognized in live-born humans. Trisomy of the sex chromosomes is also possible, for example , , and . 4/5
The risk of Trisomy 21 increases with maternal age with the risk being 1/2000 (0.05%) at age 20 increasing to 1/100 (1%) at age 40. [21] This disease can be detected by non-invasive as well as invasive procedures prenatally. Non-invasive procedures include scanning for fetal DNA through maternal plasma via a blood sample. [22]
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