Down syndrome: Causes, symptoms, and diagnosis

Down syndrome is a chromosomal disorder caused when an error in cell division results in an extra 21st chromosome.

There can be impairments in cognitive ability and physical growth, mild to moderate developmental disabilities, and a higher risk of some health problems.

Through a series of screenings and tests, Down syndrome can be detected before or after birth.

The likelihood having Down syndrome is around 1 in every 700 pregnancies. It is determined by many factors, but research suggests there is a higher risk if the mother delivers at over 35 years of age.

Before the age of 30 years, fewer than one in 1,000 pregnancies will be affected by Down syndrome. After the age of 40 years, this figure rises to about 12 in 1,000.

Fast facts on Down syndrome

Here are some key points about Down syndrome. More detail is in the main article.

Older women are more likely to give birth to a child with Down syndrome.

Normally there are two copies of every chromosome. In Down syndrome, there are three copies, either complete or partial, of chromosome 21.

The characteristics of Down syndrome include low muscle tone, short stature, flat nasal bridge, and a protruding tongue.

Individuals with Down syndrome have a higher risk of some diseases including Alzheimer’s disease and epilepsy.

Screening tests can be used during pregnancy to estimate the probability that a child will have Down syndrome.

What is Down syndrome?

[Baby with Down syndrome]
Down syndrome, also known as trisomy 21, is a genetic disorder.

Down syndrome is a genetic condition that results when there is an extra copy of a specific chromosome, chromosome 21.

It is not an illness but a term that describes the features resulting from this change.

The extra chromosome can affect the physical features, intellect, and overall development of an individual.

It also increases the likelihood of some health problems.


Down syndrome happens when there is an extra copy of genetic material on all or part of the 21st chromosome.

Every cell in the body contains genes that are grouped along chromosomes in the cell’s nucleus. There are normally 46 chromosomes in each cell, 23 inherited from the mother and 23 from the father.

When some or all of a person’s cells have an extra full, or partial, copy of chromosome 21, the result is Down syndrome.

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People with Down syndrome often have distinct physical features, unique health issues, and variability in cognitive development.

Physical features

Physical characteristics include:

eyes that have an upward slant, oblique fissures, epicanthic skin folds on the inner corner, and white spots on the iris

low muscle tone

small stature and short neck

flat nasal bridge

single, deep creases across the center of the palm

protruding tongue

large space between large and second toe

a single flexion furrow of the fifth finger

Developmental delays

Individuals with Down syndrome usually have cognitive development profiles that suggest mild to moderate intellectual disability. However, cognitive development and intellectual ability are highly variable.

Children with Down syndrome often reach developmental milestones later than their peers.

There may be a delay in acquiring speech. A child may need speech therapy to help them gain expressive language.

Fine motor skills may also be delayed. They can take time to develop after gross motor skills have been acquired.

On average, a child with Down syndrome will:

sit at 11 months

crawl at 17 months

walk at 26 months

There may also be problems with attention, a tendency to make poor judgments, and impulsive behavior.

However, people with Down syndrome can attend school and become active, working members of the community.

Health issues

Sometimes, there are general health problems that can affect any organ system or bodily function. Around half of all people with Down syndrome have a congenital heart defect.

There may also be a higher risk of:

respiratory problems

hearing difficulties

Alzheimer’s disease

childhood leukemia


thyroid conditions

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