Down syndrome is a chromosomal condition that occurs when an error in cell division results in an extra chromosome 21.

Down syndrome can affect a person's cognitive ability and physical growth, cause mild to moderate developmental issues, and present a higher risk of some health problems.

Using a series of screenings and tests, healthcare professionals can detect Down syndrome before or after birth.

Down syndrome occurs in around 1 in every 700 pregnancies. It is determined by many factors, but research suggests that there is a higher chance if the mother is older than 35 years of age.

Before the age of 30, Down syndrome occurs in fewer than 1 in 1,000 pregnancies. After the age of 40, 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.

  • Women older than 35 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, a flat nasal bridge, and a protruding tongue.
  • People with Down syndrome have a higher risk of some conditions, including Alzheimer's disease and epilepsy.
  • Doctors can use screening tests during pregnancy to estimate the probability that a child will have Down syndrome.

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Down syndrome, or trisomy 21, is a genetic condition.

It is a genetic condition that occurs when there is an extra copy of a specific chromosome: chromosome 21.

Down syndrome is not an illness. The term describes the features resulting from this change.

The extra chromosome can affect a person's physical features, intellect, and overall development.

It also increases the likelihood of some health problems.

All cells in the body contain genes that are grouped along chromosomes in the cell 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, Down syndrome occurs.

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

Physical features

Some physical characteristics of Down syndrome include:

  • eyes that slant upward, have oblique fissures, have epicanthic skin folds on the inner corner, and have white spots on the iris
  • low muscle tone
  • small stature and a short neck
  • a flat nasal bridge
  • single, deep creases across the center of the palms
  • a protruding tongue
  • a large space between large and second toe
  • a single flexion furrow of the fifth finger

Developmental delays

People who have 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 a little later than their peers.

For example, there may be a delay in learning to talk. 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 the child acquires gross motor skills.

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, most people with Down syndrome can attend school and become active, valued 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
  • epilepsy
  • thyroid conditions

Despite this, there seems to be a lower risk of hardening of the arteries, diabetic retinopathy, and most kinds of cancer.

People with a higher chance of having a child with Down syndrome might receive screening and diagnostic tests.

Screening tests can estimate the probability of Down syndrome being present. Some diagnostic tests can definitively tell whether a baby will have the condition.

Screening tests

Women aged 30–35 years or above might receive genetic screening during pregnancy. This is because the chance of having a child with Down syndrome increases as women age.

Screening tests include:

  • Nuchal translucency testing: At 11–14 weeks, an ultrasound can measure the clear space in folds of tissue behind the neck of a developing fetus.
  • Triple screen or quadruple screen: At 15–18 weeks, this measures the quantities of various substances in the mother's blood.
  • Integrated screen: This combines results from first-trimester blood and screening tests, with or without nuchal translucency, with second-trimester quadruple screening results.
  • Cell-free DNA: This is a blood test that analyzes fetal DNA present in the mother's blood.
  • Genetic ultrasound: At 18–20 weeks, doctors combine a detailed ultrasound with blood test results.

Screening is a cost-effective and less invasive way to determine whether more invasive diagnostic tests are needed.

However, unlike diagnostic tests, they cannot confirm whether Down syndrome is present.

Diagnostic tests

Diagnostic tests are more accurate for detecting Down syndrome. A healthcare professional will usually perform such tests inside the uterus.

However, they increase the risk of miscarriage, fetal injury, and preterm labor.

Diagnostic tests include:

  • Chorionic villus sampling: At 8–12 weeks, a doctor might obtain a tiny sample of placenta for analysis, using a needle inserted into the cervix or the abdomen.
  • Amniocentesis: At 15–20 weeks, they may obtain a small amount of amniotic fluid for analysis, using a needle inserted into the abdomen.
  • Percutaneous umbilical blood sampling: After 20 weeks, the doctor may take a small sample of blood from the umbilical cord for analysis, using a needle inserted into the abdomen.

A healthcare professional can also diagnose Down syndrome after a baby is born by inspecting their physical characteristics, blood, and tissue.

There is no specific treatment for Down syndrome. People who have the condition will receive care for any health problems, as other people do.

However, healthcare professionals may recommend additional health screening for issues common to the condition.

Early intervention can help a person maximize their potential and prepare them to take up an active role in the community.

Physicians, special educators, speech therapists, occupational therapists, and physical therapists, as well as social workers, can all help. The National Institute for Child Health and Human Development urge all specialists to provide stimulation and encouragement.

Children with specific learning and development difficulties may be eligible for educational support, either in a mainstream or specialized school.

In recent years, the tendency has been to attend mainstream schools, often with additional support to help them integrate and progress.

Some children will make use of an Individualized Education Program (IEP), which various specialists will support.

The most common form of Down syndrome is called trisomy 21. This is a condition wherein people have 47 chromosomes in each cell instead of 46.

An error in cell division called nondisjunction causes trisomy 21. This error leaves a sperm or egg cell with an extra copy of chromosome 21 before or at conception. This variant accounts for 95 percent of Down syndrome cases.

The remaining 5 percent of Down syndrome cases are due to conditions called mosaicism and translocation.

Mosaic Down syndrome occurs when some cells in the body are normal, while others have trisomy 21.

Robertsonian translocation occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome, usually chromosome 14. The presence of this extra part of chromosome 21 causes some Down syndrome characteristics.

A person with a translocation does not have any special physical features, but they are more likely to have a child with an extra chromosome 21.

A person with Down syndrome can do many of the things that other people do. Children may take longer to acquire skills such as walking and talking, but with stimulation, they can acquire key life skills and attend school and, in some cases, college.

Depending on how the condition affects a person, it is often possible for someone to work and to live semi-independently with Down syndrome.

People with Down syndrome need friendships and relationships. Some will live with a partner or get married, and they will have an independent life.

It is worth noting that if one partner has Down syndrome, there is a 35–50 percent chance that their children will, too.

Average life expectancy is around 60 years, but many people with Down syndrome now live into their 70s.