About Down Syndrome

There are a few basic facts you should know about Down syndrome.

It isn’t a disease.

It is a chromosome variation and has no known cause or cure.


It also affects people of all races and economic levels.

There are three types:

Trisomy 21, Translocation and Mosaicism

Trisomy 21 is the cause of 95 percent of the cases.

It occurs when the 21st chromosome separates improperly at conception. Individuals with Trisomy 21 have an extra copy of chromosome 21 in all their cells.

Translocation affects 3 to 4 percent of the people with Down syndrome.

It occurs when an extra chromosome 21 is attached to another chromosome. It’s the only inherited form.

Mosaicism, the rarest form, accounts for 1 to 2 percent of all cases.

Mosaicism occurs when the improper division happens after fertilization. So, people with mosaicism have 46 chromosomes in some cells, 47 in others.


So, it is not a disease, but a set of some 50 symptoms and markers common among people with the extra genetic material.

Few individuals have all; many have few.

That means the impact is highly variable, resulting in a wide range of cognitive ability.


Virtually all people with Down syndrome have some form of cognitive impairment.

In fact, it is one of the most common causes of that kind of disability. The incidence is about one in every 700 children born in the United States, and it affects about 300,000 families.

Finally, 80 percent of babies with Down syndrome are born to women under age 35.

Many think that it is most common among children of older moms, and it’s true, the chance a baby will have Down syndrome goes up as the mother’s age increases.

That suggests most babies with Down syndrome are born to mothers 35 or older, but pregnancy, is more common among younger women.


So while the incidence is lower, the number of babies with Down syndrome born to younger moms is higher.

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