Down syndrome is the most common chromosomal condition in the United States, occurring in about 1 in every 700 babies. In this article, Ability Central will cover the seven quick facts you need to know about the condition, including:

• What is Down syndrome?
• What are the types of Down syndrome?
• What causes Down syndrome?
• What are the physical characteristics of Down syndrome?
• What other health conditions often accompany Down syndrome?
• How is Down syndrome diagnosed?
• Where can I get more information about the disease?

What is Down syndrome?

Down syndrome is named after the English physician John Langdon Down who first described the condition in 1866. The genetic condition causes physical and developmental disorders. 

Down syndrome occurs through a process called non-disjunction, where the two copies of chromosome 21 do not separate during egg formation. If fertilized, the resulting baby has three copies of chromosome 21 in each cell. This extra chromosome leads to a range of issues that affect a person both mentally and physically. It also commonly causes other medical issues, including heart and gastrointestinal disorders.

What are the types of Down syndrome?

There are three types of Down syndrome. 

• About 95% of people with Down syndrome have Trisomy 21.  With this type of Down syndrome, each cell in the body has three separate copies of chromosome 21 instead of the usual two copies.
• Mosaicism (or mosaic Down syndrome) is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. The cells with 47 chromosomes have an extra copy of chromosome 21. Mosaic down syndrome accounts for 2% of all cases.
• Translocation Down syndrome occurs when an extra part of whole chromosome 21 is present but is attached to a different chromosome rather than a separate chromosome 21.

What causes Down syndrome?

The cause of the extra full or partial copy of chromosome 21 is unknown. 

According to the National Down Syndrome Society (NDSS), maternal age is the only factor linked to an increased chance of having a baby with Trisomy 21 or Mosaicism. 

Statistically:

• A 35-year-old woman has about a one in 350 chance of conceiving a child with Down syndrome.
• This chance increases gradually to 1 in 100 by age 40. 
• At age 45, the risk is 1 in 30. 

The mother's age does not seem to be linked to the risk of translocation Down syndrome. Instead, translocation can be caused by extra genes in the egg or sperm of one of the parents, or it can happen by chance.

What are the physical characteristics of Down syndrome?

Some common physical features of Down syndrome include:

• A flattened face
• Almond-shaped eyes 
• Eyes that slant up
• A shorter neck
• Smaller ears
• A larger tongue that tends to stick out of the mouth
• Tiny white spots on the iris of the eye
• Smaller hands and feet
• Poor muscle tone or loose joints
• Shorter in height

What health issues often accompany Down syndrome?

Some of the more common health problems among children with Down syndrome include:

• Hearing loss
• Obstructive sleep apnea
• Ear infections
• Eye diseases
• Heart defects present at birth

Many of these, including hearing loss and eye diseases can have a profound effect on a person’s ability to communicate effectively. This can greatly reduce independence for someone with Down syndrome, and often lead to exclusion and discrimination. 

How is Down syndrome diagnosed?

There are screening and diagnostic tests available to the mother during pregnancy. Screening tests estimate the likelihood that the child will be born with Down syndrome, and diagnostic tests confirm if the baby has Down syndrome. In addition, there are tests that can be run after the child is born.

See Down Syndrome: Symptoms and Early Signs to learn more about screening and testing options.

Where can I get more information about Down syndrome?

Ability Central offers a series of articles to further your knowledge about Down syndrome. See:

• Down Syndrome: Symptoms and Early Signs
• Down Syndrome: First Steps After Diagnosis
• Down Syndrome: Planning for Long-term Care