What Is Down Syndrome?

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Down syndrome, also known as trisomy 21, is a genetic disorder that occurs when a person has a full or partial extra copy of chromosome 21. It's characterized by a variety of distinctive physical features, an increased risk of certain medical problems, and varying degrees of developmental and intellectual delays. According to the National Down Syndrome Society (NDSS), approximately 6,000 babies in the United States are born with the disorder each year.

If you've just found out that your own baby has Down syndrome, you probably have myriad questions and concerns. You may even be a little scared about how you will care for a child who's likely to have many physical and intellectual challenges throughout life.

However, since the disorder was first described by a British physician named John Langdon Down in 1866, medical researchers have learned much about Down syndrome, from understanding the wide variety of symptoms and characteristics it causes to developing therapies and educational approaches to help those with it.

In order for a child with Down syndrome to reach their full potential, early intervention is key. As a parent, the more you learn about Down syndrome, and the earlier you learn it, the better equipped you will be to set your child (and yourself) up for success.

Down Syndrome (Trisomy 21)
Verywell / Emily Roberts


Every cell in the human body originates from one fertilized egg, or zygote, that duplicates itself again and again to create the different cells and tissues of the body. To understand Down syndrome, it helps to have some basic knowledge about chromosomes and genes. 

Chromosomes, as well as the genes located on them, come in pairs. Each chromosome contains the genes that provide specific information about the body—from eye color and potential height to traits such as dimples and risk of certain diseases.

What Causes Down Syndrome?

Normally, human beings inherit 23 pairs of chromosomes from both parents for a total of 46. However, people with Down syndrome wind up with 47 chromosomes because they get an extra copy of chromosome 21. This happens when the 21st pair of chromosomes from either the egg or sperm fail to separate.

Types of Down Syndrome

Although the umbrella clinical term for Down syndrome is trisomy 21, this term actually refers to one of three types of chromosome 21 anomalies. All three types of Down syndrome are genetic conditions, but only 1 percent of Down syndrome cases are passed from parent to child through genes.

The three types of Down syndrome are:

  • Complete trisomy 21: This is the most common chromosomal abnormality seen in children. It accounts for 95 percent of Down syndrome cases. The extra chromosome comes from the mother 88 percent of the time and from the father 12 percent of the time.
  • Translocation trisomy 21: This occurs when two chromosomes, one of which is a number 21, join together at the ends, creating two independent number 21 chromosomes as well as a number 21 chromosome attached to another chromosome. Approximately two thirds percent of translocations happen by chance. The rest are inherited from a parent known as a "balanced carrier." Although translocation Down syndrome occurs via a different mechanism than trisomy 21, the physical and features that mark the condition are the same.
  • Mosaic trisomy 21: In this rare form of Down syndrome, which accounts for just 2 percent to 3 percent of cases, only some cells have an extra copy of chromosome 21. Individuals with mosaic Down syndrome can have all the features of full trisomy 21, none of the features, or fall somewhere in between.

Down Syndrome Symptoms

Down syndrome is associated with a host of symptoms and characteristics. Not everyone with the condition will have all of them, but there are certain ones that will affect the majority of people who have the disorder. 

Physical Features

The most obvious characteristics of Down syndrome are physical features that usually are quite easy to recognize, including small, upturned, almond-shaped eyes, small facial features, a large tongue that may protrude slightly, and a short stocky build. Almost all infants with Down syndrome have low muscle tone, or hypotonia, which can slow reaching certain physical milestones. Hypotonia also can contribute to orthopedic problems such as unstable joints and an upper neck abnormality called atlantoaxial instability. 

Many people with Down syndrome deal with a host of medical issues throughout their lives. Common ones range from problems with hearing and vision to heart defects. Others include gastrointestinal defects and thyroid disease. Children with Down syndrome have about a 2-3% risk of leukemia (a type of cancer that affects white blood cells).

Developmental and Learning Delays

All people with Down syndrome have some degree of intellectual disability or developmental delay, which means they tend to learn slowly and may struggle with complex reasoning and judgment. There's a common misconception that children with Down syndrome have predetermined limits in their ability to learn, but this is entirely false. It's impossible to predict the degree of intellectual disability an infant with Down syndrome will have. 

Personality, Behavioral, and Psychological Traits 

A common stereotype about Down syndrome is that people who have the disorder are always happy and sociable. In fact, they experience the same full range of emotions that others do. The condition also is linked to high rates of anxiety disorders, depression, and obsessive-compulsive disorder.

People with Down syndrome are sometimes regarded as stubborn due to an innate need to have routine and sameness in order to make sense of the complexities of daily life, and they are given to self-talk as a way of processing information.


Down syndrome is caused by a chromosomal anomaly. It's estimated that there are 400 genes on chromosome 21, which basically means that people with Down syndrome have 400 "extra instructions." While this sounds like a good thing, it actually is like adding extra and unnecessary ingredients to a recipe: You will still get the same basic dish, but with notable changes. 

Usually, this happens quite randomly. However, there are three known risk factors for Down syndrome.

Maternal age is one. The chances of having a child with Down syndrome increases as a woman gets older.

Maternal age and Down syndrome risk:

  • Age 35: Risk is 1 in 385
  • Age 40: Risk is 1 in 106
  • Age 45: Risk is 1 in 30

In addition, a woman who has one child with Down syndrome is more likely to have another child with the disorder. Finally, a family history of Down syndrome can be associated with increased risk of the disorder.


An infant with Down syndrome can be identified at birth or very shortly after based on distinct physical characteristics such as a smaller-than-normal head, upward-slanting eyes, a small mouth with a protruding tongue, and a single crease (rather than two) across the palm of the hand, and a large space between the big toe and the second one.

Infants with Down syndrome also tend to be "floppy," meaning they have low muscle tone (hypotonia). And some are born with certain serious health problems that are associated with Down syndrome, such as heart defects and gastrointestinal defects. 

In order to confirm that a child with such features has Down syndrome, a chromosome analysis, called a karyotype, can be done using a blood sample from the baby. 

But testing is such that Down syndrome may also be identified (or at least suspected) before birth. Prenatal screenings and tests that can help predict that a baby will be born with Down syndrome include:

  • Ultrasound: Also called a sonogram, this imaging test of a developing fetus sometimes picks up subtle physical signs that point to an increased risk of Down syndrome.
  • Maternal serum screening: A test of an expectant mom's blood between the 13th and 28th weeks of pregnancy, the quadruple screen can suggest that a fetus is at risk of Down syndrome. It's not a definite diagnosis, however.
  • Amniocentesis: This test involves using a sample of amniotic fluid to create a karyotype. If an extra number 21 is present, then Down syndrome is diagnosed. An amnio is done between 15 and 20 weeks.
  • Chorionic villi sampling (CVS): As with amnio, CVS testing uses karyotyping to diagnose Down syndrome. However, the cells examined are taken from the placenta rather than the amniotic sac. CVS is performed at 9 to 11 weeks of pregnancy.


There's no cure for Down syndrome. From infancy on, every person with the condition will have different physical, intellectual, and emotional issues that will need to be dealt with on an individual basis, typically with the help and support of a team of doctors, therapists, and social workers.

Down Syndrome Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Mom and Baby

People with Down syndrome who are born with or develop health issues such as hypothyroidism, heart defects, or gastrointestinal abnormalities can be treated for those conditions just as anyone would be—with thyroid medication to bring levels of the thyroid hormone to normal, for instance, or with surgery to repair a hole in the heart. Likewise, children who have vision problems can have their eyesight corrected with glasses, and those with hearing loss can be fitted with hearing aids.  

In addition to medical treatment, people with Down syndrome can benefit from physical therapy, speech therapy, occupational therapy, or other interventions that target specific challenges.

Some children need help dealing with emotional and behavioral issues, such as acting out when they have trouble communicating, and can be helped by a psychologist or other mental health expert.

Finally, there are increasing numbers of assistive devices that can be helpful to people with Down syndrome, according to the NIH. Examples include devices that amplify sound, computers with touchscreens or large-letter keyboards, and even special pencils to make it easier to write. 


Learning that your baby has Down syndrome can cause a flood of emotions, and there's no question you and your family will be up against some tough challenges. In order to take the very best care of your child, you'll need to also take care of your own health so that you have plenty of strength and stamina. Getting settled into your new reality can take time—allow yourself to adjust and learn.

One of the best ways to cope with Down syndrome is to seek out other parents of children with the disorder. You could join a local support group that meets in person or find one online.

Other strategies for coping with Down syndrome include learning to "live in the moment," as suggested by the NDSS, and focusing on all the wonderful things that you and your child can do and enjoy together.

A Word From Verywell

Most people with Down syndrome lead happy and purposeful lives. Many are able to have jobs, live independently, and have relationships. In 1983, life expectancy for people with Down syndrome was age 25. Today, those with the disorder live to an average age of 60. Part of your job as a parent will be to prepare your child with Down syndrome to be as independent as possible. Depending on his level of disability, that could mean setting him up to live alone or with friends, or in a group home that will provide continued support.

But if you're just starting this journey, remember that the road ahead can't be exactly mapped. Learn as much as you can, take cues from your child, and most importantly, take it day by day.

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