What Is Down Syndrome?

Trisomy 21

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Down syndrome is a lifelong genetic condition that begins to have effects before birth and can significantly impact many aspects of a person’s life.

People who have Down syndrome can experience physical effects as well as cognitive challenges. However, the severity of the condition varies from person to person, and many people living with Down syndrome can lead happy, productive, and healthy lives.

This article describes the types of Down syndrome, its causes, risk factors, symptoms, screening tests, and long-term outlook.

Parent in kitchen holding child who has Down syndrome

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Types of Down Syndrome

Down syndrome is congenital, which means that it is present at birth. In fact, Down syndrome is present at conception. The condition occurs when a fertilized egg has an extra copy of chromosome 21. Down syndrome is also called trisomy 21.

The types of Down syndrome differ based on chromosomal patterns, as follows:

  • Trisomy 21: This is the most common type of Down syndrome. It occurs when a person has three copies of chromosome 21 in each cell of their body. A person's typical number of chromosomes is 46 (in 23 pairs). A person with Down syndrome has 47 chromosomes if all other chromosome pairs are typical.
  • Mosaic Down syndrome: This type of Down syndrome occurs when there is a mixture of some cells in the body with trisomy 21 and some cells in the body without an extra chromosome 21. The symptoms can be similar to symptoms of full trisomy 21, but sometimes the effects are milder. It is seen in about 2% of people diagnosed with Down syndrome.
  • Translocation: This type occurs when a person has extra chromosome 21 genetic material attached to another chromosome. With this type of Down syndrome, the person may have 46 chromosomes in their cells. About 3% of people diagnosed with Down syndrome have this type.

Causes and Risk Factors

The extra genetic material from the third copy of chromosome 21 causes the body to develop differently. This occurs whether a person has full trisomy 21, mosaic Down syndrome, or translocation.

This produces changes in the developing fetus's physical features. Many of the changes are present at birth, and some can develop as the child grows into adolescence and adulthood. 

Risk factors for Down syndrome include:

  • Advanced age of the birthing parent, especially age 35 and older at the time of conception
  • A family history of Down syndrome or another chromosomal disorder in a parent or sibling

Down Syndrome and Genetics

The genetic pattern of Down syndrome occurs due to the presence of an extra copy of chromosome 21 in the parents' egg cell or sperm cell. A child should normally receive only one copy of each chromosome from each parent—resulting in two copies of each chromosome in every one of the child’s cells.

When cells from either parent have two copies of any chromosome, this results in trisomy (the presence of a third copy) in all of the growing baby’s cells throughout their life. Trisomy cannot be repaired with any type of medical intervention. 

The process that causes an egg cell or a sperm cell to have an extra copy of a chromosome is called nondisjunction. It occurs during the formation of the egg and the sperm cell, prior to the embryo's conception. 

Down syndrome is not usually inherited—most people who have Down syndrome are the first in the family to have it, and their parents do not have the condition. However, it can be passed from parent to child if a person who has Down syndrome becomes pregnant or impregnates someone.

How Common Is Down Syndrome?

Down syndrome affects approximately 1 out of every 675 live births.

What Are the Symptoms of Down Syndrome?

Several characteristic physical changes and symptoms occur due to Down syndrome. These changes are often recognizable at birth, but some children might not have obvious features until early childhood.

Many of the physical features—short stature, heavy build, and prominent eyelids—might resemble other family members who do not have Down syndrome, potentially making the condition less recognizable.

Effects of Down syndrome include:

  • Skeletal differences
  • Short stature 
  • Heart malformations
  • Abnormal lung development 
  • Intestinal malformations 
  • Learning difficulties 
  • Weak immune system
  • Hypothyroidism (low function of the thyroid gland)

People who have Down syndrome have a higher-than-average risk of developing Alzheimer’s dementia later in life. Sometimes people who have Down syndrome have blood cell abnormalities affecting white blood cells and red blood cells.

Screening for Down Syndrome During Pregnancy

It is possible to identify Down syndrome during pregnancy. Testing and screening are not a standard part of prenatal care, but they can be done at the pregnant person's request.

Sometimes people who are at risk of having a baby with Down syndrome may request a screening test during pregnancy. But some people do not want to screen, even if they know that their baby is at high risk. And some people may ask for a screening test even when they do not have an elevated risk. 

First Trimester

A pregnant person can have a quad screen early in pregnancy. This blood test measures hormones in the pregnant person’s blood that might be abnormal if the growing baby has Down syndrome or other congenital problems. The test cannot rule in or rule out Down syndrome.

Second Trimester

Ultrasound testing can examine the fetus’s physical features, potentially identifying some characteristics that can occur with Down syndrome—such as heart malformations. This test can reliably identify developmental differences, but it does not rule in or rule out Down syndrome.

A chromosomal examination can be done with amniocentesis or chorionic villi sampling. These minimally invasive procedures involve collecting a sample of cells that are genetically identical to the baby’s cells. The cells are collected with a needle using ultrasound guidance.

This test can definitively diagnose Down syndrome and specifically identify the type of Down syndrome.

Living With Down Syndrome

Living with Down syndrome is a challenge for the whole family. Accommodations are often necessary to achieve learning goals and physical development during the toddler and school-age years. Many school districts offer accommodations for people who are living with Down syndrome,

Additionally, parents should seek the assistance of a multidisciplinary healthcare team to get the testing, therapy, and assistive devices needed to optimize quality of life.

Socializing is possible for people who have Down syndrome. Many people with Down syndrome can develop friendships and a supportive community.

People with Down syndrome can enjoy hobbies and other interests and often pursue those interests with lessons or classes. Many people with Down syndrome also have talents that they work to improve, such as art, music, acting, and more.

Complications of Down Syndrome

Several complications can occur as a result of having Down syndrome. These are not caused directly by the chromosomal abnormality, but they can develop due to the physical changes that occur as a result of the chromosomal abnormality.

Examples of complications include:

  • Scoliosis: This abnormal curve of the spine can develop due to spine differences and altered body positioning.
  • Heart failure: The heart does not pump enough blood to meet the body's needs. Can be a result of heart defects, high body weight, and low physical activity
  • Bowel obstruction: A blockage of the intestine can occur due to intestinal malformation and dietary factors.
  • Lung aspiration: Drawing foreign substances into the lungs may occur due to contributing factors such as lung disease, scoliosis, and general weakness.
  • Depression: This mood disorder can occur as a result of the impact of coping with physical and cognitive limitations.
  • Infections: People who have Down syndrome can have a higher risk of infections, including severe effects of COVID-19.

Long-Term Outlook for Down Syndrome

In general, many people with Down syndrome can live long and healthy lives. The life expectancy is improving and is now over age 55.

Many people with Down syndrome can work in a job compatible with their physical and cognitive abilities. Support groups and advocacy organizations can often help find resources for job placement, recreational activities, transportation, and financial aid. 

In general, people with Down syndrome are not able to live independently. Some may live with their families. Others may live in a group home or assisted living facility equipped to support their limitations and provide appropriate day-to-day help. 

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.