Trisomy 21, also known as Down syndrome, is a genetic disorder that occurs when a baby has an extra copy of chromosome 21. Typically, a baby is born with 46 chromosomes, but babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. This extra copy changes how the babys body forms and functions as it grows during pregnancy and after birth. Down syndrome is the most common chromosomal anomaly in humans and can cause intellectual disabilities and health issues. There are three types of Down syndrome:

• Trisomy 21: About 95% of people with Down syndrome have Trisomy 21. With this type of Down syndrome, each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2 copies.
• Translocation Down syndrome: This type accounts for a small percentage of people with Down syndrome (about 3%). This occurs when an extra part or a whole extra chromosome 21 is present, but it is attached or “trans-located” to a different chromosome rather than being a separate chromosome 21.
• Mosaic Down syndrome: This type occurs in about 2% of cases and happens when an error in cell division happens after the egg is fertilized. People with this syndrome have both normal cells and some cells with an extra chromosome number 21.

Trisomy 21 can be diagnosed prenatally through direct analysis of fetal chromosomes, by karyotype or DNA microarray, obtained from amniocentesis, chorionic villus sampling, or percutaneous umbilical blood sampling. Clinical diagnosis of Down syndrome can be confirmed by genetic testing, typically karyotype from infant’s blood or tissue.