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 varying developmental differences, and present a higher risk of some health problems.

Healthcare professionals can use a series of screenings and tests to detect Down syndrome before or after birth.

Down syndrome occurs in around 1 in every 700 babies born.

This article discusses the causes or contributing factors of Down syndrome, the characteristics of the condition, the diagnosis, types of Down syndrome, and whether this condition is genetic or linked to autism.

Fast facts on Down syndrome

  • Mothers 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 is a genetic condition that occurs when there is an extra copy of a specific chromosome: chromosome 21. The extra chromosome can affect a person’s physical features, intellect, and overall development. It also increases the likelihood of some health problems.

Many factors contribute to Down syndrome, but the prevalence is higher in older pregnant people. There may be a higher chance if a pregnant person is over age 35.

A pregnant person at age 25 has about a 1 in 1,250 chance of conceiving a child with Down syndrome. At age 40, the incidence becomes approximately 1 in 100.

All cells in the body contain genes, which have a specific code or set of instructions for creating the cells. These genes sit inside chromosomes in the cell nucleus. There are typically 46 chromosomes in each cell — 23 inherited from the mother and 23 from the father.

Down syndrome occurs when some or all of a person’s cells have an extra full or partial copy of chromosome 21.

In most cases, Down syndrome is not inherited and does not run in families. Though Down syndrome comes from the genes themselves, this is generally due to errors between a sperm and an egg, when the genetic information that forms a child first combines and copies.

Down syndrome can have links to genetics. There may be some links between parents of a person with translocation Down syndrome and their likelihood to have more children with Down syndrome in some cases.

The Genetic and Rare Diseases Information Center notes that in parents of a child with Down syndrome due to translocation, there may be an increased chance of Down syndrome in future pregnancies if one of the parents has a genetic rearrangement called a balanced translocation. However, this does not occur in every case of translocation Down syndrome.

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

Physical features

Some common physical characteristics of Down syndrome can include:

  • eyes that slant upward
  • skin folds on the inner corner of the upper eyelid
  • 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
  • small hands and feet

Developmental delays

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

People with Down syndrome also experience learning difficulties that lead to developmental delays. A person with Down syndrome has a specific pattern of cognitive and behavioral features. These differ from what is seen in typically developing children and children with other causes of intellectual disability.

Children with Down syndrome often reach developmental milestones a little later than their peers. They may be slow to sit, turn over, and stand.

There may also be a delay in coordination and fine motor skills (movements using small muscles in the hands and wrists). These skills can take time to develop after the child acquires gross motor skills, which involve movement of the whole body.

Development of speaking and grasping a language may also take longer than expected. With this said, people with Down syndrome eventually meet many of these milestones.

People with Down syndrome may also experience:

  • difficulties with attention
  • a tendency to make poor judgments
  • impulsive behavior

With engagement and regular therapy, most people with Down syndrome can attend school and become active members of the community.

Health issues

Sometimes, general health problems can affect any organ system or bodily function. About 40–60% of all people with Down syndrome have a congenital heart defect.

There may also be a higher risk of:

Children with Down syndrome are also more likely to develop some infections, such as:

There seems to be a lower risk of hardening in 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.

There are two categories of screening tests that doctors can perform.

Prenatal screens can estimate the probability of a person having a baby with Down syndrome and justify further tests, but they do not diagnose Down syndrome.

Diagnostic tests can definitively tell whether a fetus will have the condition and identify certain abnormalities.

Screening tests

Due to the increased chances of having a child with Down syndrome, people aged 30–35 or older might receive genetic screenings during pregnancy.

These tests are completely optional, and not everyone will choose genetic screening while pregnant.

There are several screening tests, which 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 test measures the quantities of various substances in the pregnant person’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 pregnant person’s blood.
  • Genetic ultrasound: At 18–20 weeks, doctors combine a detailed ultrasound with blood test results.

Screening tests cannot confirm whether Down syndrome is present.

Screening is a cost effective and less invasive way to determine whether doctors may need to order further diagnostic testing.

Diagnostic tests

Diagnostic tests are more accurate at detecting Down syndrome.

A healthcare professional will usually perform such tests inside the uterus.

However, diagnostic tests can increase the risk of:

  • miscarriage
  • fetal injury
  • preterm labor

Diagnostic tests include:

  • Chorionic villus sampling: At 9–11 weeks, a doctor might use a needle inserted into the cervix or the abdomen to take a tiny sample of placenta for analysis.
  • Amniocentesis: At 14–18 weeks, a doctor may insert a needle into the abdomen to obtain a small amount of amniotic fluid for analysis.
  • Percutaneous umbilical blood sampling: After 20 weeks, the doctor may insert a needle into the abdomen to take a small sample of blood from the umbilical cord for analysis.

A healthcare professional can also diagnose Down syndrome after a baby is born by inspecting their:

  • physical characteristics
  • blood
  • 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 people with the condition.

The National Institute of Child Health and Human Development recommends early intervention with specialized programs to help a person maximize their potential and prepare to take an active role in the community. Early intervention may help improve outcomes for people with Down syndrome.

Working with a team of specialists can provide stimulation and encouragement to the child as they grow. This can include many specialists in different fields to help the person develop. These specialists may include:

  • physicians
  • physical therapists
  • speech therapists
  • special educators
  • occupational therapists
  • social workers

Children with specific learning and developmental difficulties may be eligible for educational support in a public or specialized school.

Children with Down syndrome are entitled to an appropriate educational environment that fits their needs, often with additional support to help them integrate and make progress.

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

There are a few different types of Down syndrome:

  • Trisomy 21: This is the most common type, making up about 95% of cases. It occurs when 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.
  • Mosaic Down syndrome: This type occurs in about 2% of people with Down syndrome. Some of the children’s chromosomes will contain an extra copy of chromosome 21, while other cells will have the typical two copies. Children have fewer characteristic features of the condition, depending on the number of cells with either 2 or 3 copies of chromosome 21.
  • Translocation Down syndrome: This makes up about 3% of cases of Down syndrome. This type 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.

Down syndrome and autism spectrum disorder are two types of conditions that can cause cognitive changes in a person. These conditions do have some key differences.

Unlike with Down syndrome, there are no distinct or easily recognizable physical characteristics to identify a person with autism.

Down syndrome is a genetic condition stemming from changes to the genes themselves. Autism is a neurological condition, and the exact cause of autism is not quite clear.

Compared with an average child of a similar age, both conditions may cause communication differences or learning disabilities, and how this appears can vary in each condition and individual.

There are no cures for either condition. Most people will use many treatment and therapy methods to help manage their condition or make improvements in important areas they choose to.

A person with Down syndrome can do many things that other people do. Children may take longer to acquire skills such as walking and talking, but they can develop at their own pace and attend school with early stimulation and treatment.

Depending on how the condition affects a person, they may work and live semi-independently with Down syndrome.

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

The Centers for Disease Control and Prevention (CDC) note that the life expectancy of a person with Down syndrome has increased significantly with modern advancements in medicine and treatment. In 1960, a person with Down syndrome lived an average of 10 years. By 2007, a person with Down syndrome lived an average of 47 years.

A person born with Down syndrome today has the best chance at leading a long, engaging life due to modern advancements in healthcare, early treatments, and successfully managing congenital issues such as heart conditions.

Down syndrome occurs due to an error in chromosome 21. This error copies into the genes, creating a set of characteristics common to Down syndrome. This includes physical characteristics, developmental delays, and risks of other health conditions.

While there is no treatment for Down syndrome, early engagement can help a child develop and progress at their own pace. Modern advancements in healthcare and treatment give people with Down syndrome a better outlook than ever before, which may continue as more medical advancements occur.