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Down syndrome or Down's syndrome, [12] also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. [3] It is usually associated with developmental delays, mild to moderate intellectual disability , and characteristic physical features.
Types of chromosome problems that are occasionally seen include trisomy 13 (Patau syndrome) or trisomy 21 (Down syndrome). [1] In the case of an isolated EIF, and no other ultrasound findings, some studies show that the risk for a chromosome abnormality is approximately two times a woman's background risk.
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] Mitotic nondisjunction after conception would lead to mosaicism, and is discussed later. Some cases have been reported of people with Down syndrome ...
In another study values of 79.6% and 2.7% for the combined screening were then improved with the addition of second trimester ultrasound scanning to 89.7% and 4.2% respectively. [13] A further study reported detection of 88% for trisomy 21 (Down syndrome) and 75% for trisomy 18 (Edwards syndrome), with a 3.3% false-positive rate. [14]
Chromosome 21 is both the smallest human autosome and chromosome, [4] with 46.7 million base pairs (the building material of DNA) representing about 1.5 percent of the total DNA in cells. Most people have two copies of chromosome 21, while those with three copies of chromosome 21 (trisomy 21) have Down syndrome.
This is a list of people with Down syndrome, a condition also known as Down's syndrome or trisomy 21. Down syndrome is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. [1] It is typically associated with developmental delays, characteristic physical features, and mild to moderate intellectual disability ...
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 ...
Although 91% of fetuses affected by Down syndrome exhibit this defect, 5% of fetuses flagged by the test do not have Down syndrome. Ultrasound may also detect fetal organ anomaly. Usually scans for this type of detection are done around 18 to 23 weeks of gestational age (called the "anatomy scan", "anomaly scan," or "level 2 ultrasound").