A Guide to Down Syndrome

Updated July 28, 2020

This article was scientifically reviewed by YourDNA

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A list of references is also included at the bottom of this article.

What are Trisomies?

To better understand the origins of Down syndrome, it is essential to become familiar with the role that trisomies play in this condition.

A trisomy is a type of aneuploidy. In humans, aneuploidy occurs when an individual has either extra or missing chromosomes; specifically, a trisomy is a condition where there are three copies of the same chromosomes rather than the typical two copies 1.

What's in this Guide?

Disclaimer: Before You Read

It is important to know that your genes are not your destiny. There are various environmental and genetic factors working together to shape you. No matter your genetic makeup, maintain ideal blood pressure and glucose levels, avoid harmful alcohol intake, exercise regularly, get regular sleep. And for goodness sake, don't smoke.

Genetics is a quickly changing topic. Read More...

Our bodies are made up of millions of cells, and in each of these cells we have 23 pairs of chromosomes, or 46 total. In each pair, one chromosome is from our mother and the other is from our father.  We all begin as one single cell at conception, but as we develop, that cell divides into two, and then those cells divide, and so all of our cells continue to divide throughout our life as we grow. Cell division is facilitated by our chromosomes, and our chromosomes undergo a process called replication where they copy themselves and then divide as well, so that way each of our cells has its own set of chromosomes. If the chromosome pairs do not separate properly during meiosis, or sex cell division, egg or sperm cells may end up with an extra copy of one of the chromosomes 2.

There are two main kinds of trisomies:

  • Full trisomy, which occurs when an entire extra chromosome is present 3.
  • Partial trisomy, which occurs when only a part of an extra chromosome is present rather than the entire chromosome 4.

Trisomies can occur in any chromosome. The signs and symptoms of a patient will depend on which chromosome has an extra copy. Often, trisomies result in miscarriage instead of a live birth 5.

Of the 23 pairs of chromosomes, the first 22 pairs are considered to be autosomal chromosomes, which determine a range of characteristics like hair color or height, and the last pair is considered to be a pair of sex chromosomes, which determines sex. Trisomies of the first 22 pairs of chromosomes can be characterized as autosomal trisomies, also known as trisomies of the non-sex chromosomes. When this happens, they are described by referencing the specific chromosome where the trisomy occurred. For example, the presence of an extra chromosome 21, found in Down syndrome, is also known as Trisomy 21 6.

The most common types of autosomal trisomies that survive to birth in humans are 7:

  • Trisomy 21 (Down syndrome)
  • Trisomy 18 (Edwards syndrome)
  • Trisomy 13 (Patau syndrome)

What is Down Syndrome?

Down syndrome occurs when an error in cell division results in the presence of an extra chromosome 21.

Usually there are 23 pairs of chromosomes, or 46 total chromosomes, in every cell. Of these, 23 chromosomes are inherited from the mother, and 23 chromosomes are inherited from the father.

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

A person who has Down syndrome can have impacted physical growth and cognitive abilities, and mild to moderate developmental issues 8. There is also a higher risk of some health problems such as Alzheimer's disease, heart disease, gastrointestinal disorders, and epilepsy 9.

Overall, Down syndrome occurs in about 1 in every 700 pregnancies 10. Certain factors may increase or decrease the risk of having a child with Down syndrome, specifically age.

It is known that there is a higher propensity for Down syndrome in a child if a mother is older than 35 years of age 11. Before age 30, Down syndrome appears in fewer than 1 in every 950 pregnancies 12. After age 40, this figure rises to about 1 out of every 100 pregnancies, and that risk continues to increase with maternal age 13. It’s important to remember that while the risk of having a child with Down syndrome is higher in women over the age of 35, the majority of individuals with Down syndrome are born to women younger than 35 simply because women under the age of 35 are having more children 14

Doctors can screen for Down syndrome both during pregnancy and after birth. 

Characteristics of Down Syndrome

There are several distinct physical features and developmental issues associated with someone who has Down syndrome.

When you think of individuals with Down syndrome, you may notice that many of them share similar physical features. Most notably are the similar facial features, which include upward slanting eyes, smaller ears, a protruding tongue, and a flatter nose 15. Individuals with Down syndrome also tend to have a lower muscle tone, a shorter stature, and smaller hands and feet 16. There are also some shared physical features that you may have not noticed; for example, many individuals with Down syndrome have a large space between their first and second toe, called a sandal gap, a single crease across their palm rather than the typical two creases, and a very small pinky finger that curves towards the thumb, called clinodactyly 17.

There are also developmental delays and intellectual disorders associated with Down syndrome. The average intelligence quotient (IQ) of individuals with Down syndrome is 50, meaning there is a moderate intellectual disability, but IQ can range from 30 to 70 18. This range highlights the fact that Down syndrome impacts individuals differently, so cognitive development can vary.  Individuals with Down syndrome also often experience developmental delay. For example, children without Down syndrome may sit around 9 months and walk around 12 months whereas children with Down syndrome may sit around 11 months and walk around 26 months 19. Individuals with Down syndrome also tend to have certain behavioral issues, such as impulsive behavior or issues with maintaining attention 20.

Related Health Issues

People with Down syndrome may also experience other health problems more frequently.

Individuals with Down syndrome experience congenital heart defects at a much higher rate – about 50% of all individuals with Down syndrome have some type of congenital heart defect. The most common heart defect is a ventricular septal defect, which is an abnormal hole present between the chambers of the heart that prevents the heart from working properly 21. There are other congenital defects that occur at a higher rate in individuals with Down syndrome as well, including gastrointestinal and respiratory defects 22

Individuals with Down syndrome also experience neurological issues at a higher rate. For example, Alzheimer’s disease and seizures occur at a much higher rate and with a much earlier age of onset 23. These neurological issues may be the cause of their endocrine issues, since the brain controls hormone release. Individuals with Down syndrome are more likely to have thyroid conditions, which can lead to obesity as well as sleep apnea 24

Causes of Down Syndrome

Down syndrome is a genetic condition, and can only be caused by a change in genetic material during or after conception. There are three main genetic mechanisms that confer Down syndrome.

Trisomy 21 is the most common type of Down syndrome, accounting for 95% of all diagnoses 25. In trisomy 21, there are three copies of the 21st chromosome instead of the typical two. This is caused by abnormal cell division that takes place during the development of the sperm cell or the egg cell 26. This means that instead of the usual 46 chromosomes, a person with trisomy 21 has 47.

A translocation accounts for 4% of all cases of Down syndrome and occurs when part of chromosome 21 is translocated, which means that an arm of the chromosome breaks off during conception or cell division and attaches to another chromosome 27. In most cases, the extra chromosomal arm attaches to chromosome 14 28. The total number of chromosomes in a person's cells remain at 46, but the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.


Mosaicism accounts for about 1% of all cases of Down syndrome. Mosaicism means that not all of the cells in our bodies hold the same genetic information. If an individual is mosaic for Down syndrome, a portion of their cells have either trisomy or a translocation while another portion of their cells may not have any genetic change at all. This occurs when there is abnormal cell division occurs in one – but not all – of the cell divisions at conception 29. As a result, there is a mixture of two types of cells. Some cells may contain the usual 46 chromosomes and others contain 47 chromosomes.

There are no behavioral or environmental factors that cause Down syndrome 30.

The Genetics of Down Syndrome

Down syndrome is a genetic disorder caused by a mistake in cell division during early human development.

Abnormal cell division results in an extra full or partial copy of chromosome 21, and it is this extra genetic material that causes the physical and developmental characteristics associated with Down syndrome 31.

Most of the time, Down syndrome is not inherited. Instead, there is usually a random, spontaneous mistake in cell division that causes Down syndrome. However, translocations can be passed from parent to child 32. This is quite rare since only about 4% of individuals with Down syndrome have a translocation 33.

Sometimes, parents may have a balanced translocation, which means there is rearranged genetic material from chromosome 21 on another chromosome, but there is no extra genetic material. Since there is no extra genetic material, these individuals will not have Down syndrome. Despite the parent having no symptoms of Down syndrome, they could pass an unbalanced translocation on to their children. An unbalanced translocation means there is extra genetic material, so those children would have Down syndrome 34.

Risk Factors for Down Syndrome

Certain risk factors place a child at higher risk of being born with Down syndrome. Those risk factors include:

  1. Maternal age. The older a woman is the greater the chances of giving birth to a child with Down syndrome. This is especially true after a woman turns 35 years old because older eggs tend to have a higher risk of improper chromosome division. This is because chromatin, a protein that covers our chromosomes, tends to abnormally clump with chromatin on other chromosomes in older egg cells, which can lead to more instances of the abnormal cell division that causes Down syndrome 35.
  2. Being carriers of the genetic translocation for Down syndrome. Both men and women with a balanced translocation can pass that translocation for Down syndrome on to their children 36.
  3. Having had one child with Down syndrome. Parents who have one child with Down syndrome are at a higher risk of having another child with Down syndrome – the recurrence rate for a reproductive couple that do not have a translocation and had a child with Down syndrome is 1-2% 37.

Diagnosis of Down Syndrome

People who have higher risk factors of having a child with Down syndrome might receive screening and diagnostic tests.

We are able to screen and diagnose Down syndrome during pregnancy or after a child is born. There are two categories of tests for Down syndrome during pregnancy: screening and diagnostic.

A prenatal screening test can show an increased likelihood that a fetus has Down syndrome, but it cannot determine whether Down syndrome is definitely present.

Prenatal screening tests for Down syndrome are conducted in several ways:

  • A blood test and an ultrasound test during the first and/or second trimester of pregnancy. This is the most conventional approach for screening during the first trimester and allows a doctor to check for markers, or certain hormones, in a mother's blood that may suggest the presence of Down syndrome. These hormones are produced normally in pregnancy, but a specific pattern of higher or lower levels may suggest Down syndrome. In conjunction with the blood test, an ultrasound can be performed to look at nuchal translucency, or a fold of skin behind the neck that occurs normally during pregnancy – a larger nuchal translucency can suggest Down syndrome 38.
  • A combined test. This combines the first and second trimester blood tests and ultrasounds together to predict a risk for the pregnancy to be affected with Down syndrome 39.
  • Non-Invasive Prenatal Screening (NIPS): NIPS is another screening test that can be done in the first or second trimesters of pregnancy. During pregnancy, some of the baby’s DNA naturally enters the mother’s bloodstream. NIPS takes a blood sample, that holds both maternal and fetal DNA, and compares how much genetic material is present between the mother and the baby. If there is a higher ratio of fetal chromosome 21, then the baby may be considered at risk of having Down syndrome 40.

When screening tests have positive results, more invasive diagnostic tests may be required. With diagnostic tests, there is about a 1 in 500 risk for miscarriage, bleeding, or infection, but a definitive diagnosis can be made 41.

Diagnostic tests include:

  • Chorionic villus sampling (CVS): CVS is completed between the 11th and 14th week of pregnancy and entails a long, thin needle being inserted through either the abdomen or the cervix to extract a tiny sample of the placenta, which holds the fetus during pregnancy. The placenta typically has the same DNA as the fetus, although there is a less than 1% chance of there being confined placental mosaicism, meaning that the placenta’s genetic information does not match the fetus’ genetic information 42.
  • Amniocentesis. Amniocentesis is completed between the 15th and 20th week of pregnancy and entails a long, thin needle being inserted through the abdomen to collect a small sample of amniotic fluid, which is the fluid that surrounds the fetus in utero. Amniotic fluid holds skin cells from the fetus that have naturally sloughed off as the fetus moves and develops, and the skin cells hold DNA that can be used for genetic testing 43.

With the DNA extracted from an amniocentesis or CVS, a karyotype is created that can show whether or not there is extra genetic material for chromosome 21. A karyotype is a photograph of your chromosomes from one cell, so clinicians are able to see whether or not there is extra genetic material from chromosome 21 present 44.

Screening and diagnostic tests cannot predict the full impact of Down syndrome on a baby. There is no way to predict the severity or specific elements of the presentation of Down syndrome before birth.

Who Should Consider Testing for Down's Syndrome?

There is no way to prevent Down syndrome. Consult with a genetic counselor if you are high risk for having a child with Down syndrome or have already had a child with Down syndrome.

Individuals who are high-risk for having a pregnancy with Down syndrome include mothers above the age of 35 and reproductive couples who have a translocation that involves chromosome 21 45. A genetic counselor will be able to provide you with important information about the risks you face, the chances of having a child with Down syndrome and what tests you may want to consider.

Treatments and Care Options for Individuals with Down Syndrome

Because each person with Down syndrome has different needs, strengths, and limitations, there is no single standard of treatment for someone with the condition.

Down syndrome is a lifelong condition and it is crucial to start treatment and services early in life to help improve a child's developmental and intellectual abilities as soon as possible.

Early intervention refers to different services and supports that families with children who have Down syndrome may use to help improve outcomes. Early intervention begins shortly after birth and often continues until a child reaches age 3. After that age, most children receive interventions and treatment through their local school district 46.

As individuals with Down syndrome age, the best way for them to develop their full potential is by receiving multi-disciplinary treatment that addresses their unique spectrum of symptoms. By doing so, they can take up an active and more independent role in the community. To promote independence and productivity, treatment services can include:

  • Physical therapy to build motor skills, increase muscle strength, and improve posture and balance. This is especially important early in a child's life when physical abilities lay the foundation for other skills 47.
  • Speech-language therapy improves communication skills and use language more effectively, offsetting the fact that Down syndrome children often learn to speak later than their peers. In many cases, children with Down syndrome understand language and want to communicate before they can speak 48. A speech-language therapist can help a child use other means of communication, such as sign language and pictures until the child learns to speak 49
  • Occupational therapy finds ways to adjust everyday tasks and conditions such as eating, getting dressed, writing, and using a computer. At the high school level, an occupational therapist could help teenagers identify jobs, careers, or skills that match their interests and strengths 50.
  • Emotional and behavioral therapies find appropriate responses to both desirable and undesirable behaviors. These therapists try to understand why a child is acting out and create strategies for avoiding or preventing these situations from occurring. The changes in hormone levels during puberty can cause more aggressive behavior and a therapist can help teenagers recognize their intense emotions and teach them healthier ways to express their frustration 51.

Children with Down syndrome may also need extra help or attention in school, although many children are included in regular classes with an aide present. Alternatively, specialized schools are available 52.

In many cases, individuals with Down syndrome can be helped by assistive devices which can be any type of material, equipment, tool, or technology that enhances learning or makes tasks easier to complete. This might include hearing aids for hearing problems, bands that help with movement, special pencils to make writing easier, and touchscreen computers or computers with large-letter keyboards 53.

In addition, individuals with Down syndrome are at a greater risk for developing health issues, and they will need to be monitored and treated specifically for those problems as they develop. For example, an infant with Down syndrome may need surgery early in life to correct a heart defect or an adult with Down syndrome may have digestive problems that require a lifelong special diet. A specialized healthcare team routinely meets with individuals with Down syndrome to maintain proper treatment 54.

Federal Law and Down Syndrome

Most children with Down syndrome are eligible for free and appropriate public education under Public Law 105-17.

The Individuals with Disabilities Education Act (IDEA) makes it possible for children with disabilities to get free educational services and devices to help them learn as much as they can. Each child is entitled to these services from birth through the end of high school, or until age 21, whichever comes first. Most early intervention programs fall under this legislation 55.

Each child must be taught in the least restrictive environment that is appropriate.

This does not mean that each child will be placed in a regular classroom. Educators must work to provide an environment that best fits the child's needs and skills. If a child qualifies for special services, a team of people must work together to design an Individualized Educational Plan (IEP) for the child.

The National Early Childhood Technical Assistance Center, run by the U.S. Department of Education, provides information and resources for parents and families looking for early intervention programs. Visit the Early Childhood Technical Assistance Center for more information.

Other early intervention resources are available at Day2DayParenting.com and at FamilyVoices.org. The U.S. Department of Education also funds the Parent Center Network, which provides resources, contacts, and assistance for parents and families trying to navigate special education programs.

The Prognosis for Down Syndrome

The individual prognosis for someone with Down syndrome varies greatly depending on the severity of the condition.

There is no cure for Down syndrome – it is a life-long condition. However, there are multiple treatment options that can be tailored for an individual’s specific set of symptoms that can help them achieve the fullest life possible.

Due to advancements in treatment, Down syndrome patients are living much longer. It is not uncommon for someone with Down syndrome to live to 55 or 60 years old, and in many cases, people live much longer 56

The quality of life for Down syndrome patients still remains difficult at times, but with a trend toward early intervention and more efforts to mainstream children when possible, integration and the ability to lead a healthier life is more widely accepted than ever.

What to do Next: Living with Down Syndrome

For additional information on Down syndrome, contact the following organizations:

Down Syndrome Research Foundation (DSRF): DSRF initiates research studies to better understand the learning styles of those with Down syndrome.

Global Down Syndrome Foundation: This foundation is dedicated to improving the lives of people with Down syndrome through research, medical care, education and advocacy.

National Association for Down Syndrome: Provides support for all persons with Down syndrome in achieving their full potential. They seek to help families, educate the public, address social issues and challenges, and facilitate active participation.

National Down Syndrome Society (NDSS): NDSS seeks to increase awareness and acceptance of those with Down syndrome.

DS-Connect®: The Down Syndrome Registry: Parents and families of children with Down syndrome can connect with other families and people with Down syndrome from around the world to learn more and share information. This safe and secure registry helps families and researchers identify similarities and differences in the symptoms and treatment of people with Down syndrome and guide future research.


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