Fact Friday: Down syndrome is NOT rare!

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“Down syndrome is a rare disorder”. 

Facts to disprove myth:

According to the National Down Syndrome Society, Down syndrome is the MOST commonly occurring chromosomal condition. Approximately one in every 700 babies in the United States is born with Down syndrome, which is around 6,000 births per year. 

What is Down syndrome?

Down syndrome is a condition that occurs when someone has an extra chromosome.

In every cell in the human body there is a nucleus, where genetic material is stored in genes. Genes carry the codes responsible for all of our inherited traits and are grouped along rod-like structures called chromosomes. Typically, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21. This affects how the baby’s body and brain develop.

What are the 3 forms of Down syndrome?

Trisomy 21:

Trisomy 21 is the most common form, accounting for around 95% of cases. The National Down Syndrome Society states that it is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body.


Translocation Down syndrome accounts for 3% of individuals with Down syndrome. An extra partial or entire chromosome is attached to a different chromosome rather than being a separate chromosome 21.


Mosaic Down syndrome affects 2% of individuals with Down syndrome. The word mosaic means mixture, meaning that some cells have 3 copies of chromosome 21 and some have 2 copies.


Centers for Disease Control and Prevention (2019). Facts about Down syndrome. Retrieved from https://www.cdc.gov/ncbddd/birthdefects/downsyndrome.html

National Down Syndrome Society (2020). What is Down syndrome. NDSS. Retrieved from https://www.ndss.org/about-down-syndrome/down-syndrome/

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