Down Syndrome: What is Down Syndrome? - Part 1

This is part 1 of a three part series on the topic of Down syndrome. Look for part 2 and part 3 coming up over the next few weeks.


Down syndrome is one of the most common causes of intellectual disability. An intellectual disability is a disability that greatly affects mental abilities and adaptive behavior. Mental abilities include learning, reasoning, planning, problem solving, and so on. Adaptive behavior includes everyday skills needed to live, work, and interact with others in the community. The symptoms of Down syndrome may be different from person to person with the most common being physical, medical conditions, intellectual and developmental symptoms, along with impairments in memory and language. Down syndrome occurs in about 1 out of 700 babies. The average lifespan of individuals with Down syndrome since the 1970s has nearly doubled from 30 to 60 years due to increased medical treatments and support programs.


Researchers continue to investigate the causes that lead to changes in chromosome 21. The causes that lead to these changes are not fully understood at this time.

Risk factors

Women 35 years of age and older who become pregnant are at a higher risk than women younger than 35 who become pregnant.

Screening and Diagnosis

Screening for Down syndrome during pregnancy is done via amniocentesis, blood work, and/or ultrasound. When Down syndrome is suspected after birth, it is confirmed with genetic testing.

Types of Down Syndrome

Research shows that Down syndrome can be caused by one of three different changes to chromosome 21. Chromosomes contain the genes responsible for telling cells how to function and multiply. Humans have 23 pairs of chromosomes with a total of 46 chromosomes in each cell. The changes to chromosome 21 that cause Down syndrome are:

  • Trisomy 21 is the most common and occurs in 95% of the Down syndrome population. Each cell has three copies of chromosome 21 instead of two copies.

  • Translocation occurs in 3.3% of the Down syndrome population. An extra chromosome 21 attaches itself to a different chromosome.

  • Mosaicism occurs in 2% of the Down syndrome population. Some cells have three copies of chromosome 21 and others have only two.

Physical Characteristics

Physical characteristics include a rounded face that tends to be flat, eyes that are upward and slanting with an eye fold, and decreased height and weight throughout development to adulthood. Children with Down syndrome are prone to developmental motor delays in all areas due to requiring additional time to learn movements as they become more complex.

Health Problems

Individuals with Down syndrome maybe at risk for the following common health problems: hearing problems, vision problems, poor muscle tone, sleep apnea, congenital heart disease, thyroid disease, dementia, and seizures.

Speech and Language Development

  • 50 to 70% of children with Down syndrome experience middle ear infections, which results in a temporary mild hearing loss. More research is needed to determine if early middle ear infections may have an effect on language development.

  • Speech intelligibility or how well words can be understood by others may be affected due to a smaller mouth cavity, enlarged tongue, and high arched palate. Other factors that affect speech intelligibility include the movement of the articulators (lips, teeth, hard and soft palates), rate, and range of motion.

  • Mental abilities may range from mild to severe with the most common areas affected include learning, long and short term memory, and language.

For additional information on Down syndrome, visit the websites of the National Institute of Health and the National Down Syndrome Society.

Ivan Campos, M.S., CCC-SLP, is a pediatric bilingual speech-language pathologist in Southern California. He can be reached at