Down syndrome is a condition that occurs when a person has an extra chromosome, specifically an extra copy of chromosome 21. People with Down syndrome have differences in physical and mental development. This article will review what Down syndrome is, the different types of Down syndrome, and how it is diagnosed.
Down syndrome happens when a person has an extra copy of chromosome 21 in their cells.
Although mothers over 35 have a higher chance of having a baby with Down syndrome, it can happen at any age, and there is no known prevention.
Down syndrome is not caused by anything parents did or did not do before or during pregnancy.
Prenatal screening tests may identify some signs of Down syndrome, but only diagnostic tests done before or after birth can provide a firm diagnosis.
People with Down syndrome will have physical and mental differences, including differences in cognitive ability.
Genetics and Down syndrome
To understand Down syndrome, it is helpful to understand a little bit about genetics. Each cell in our body has genetic information stored in our chromosomes. Chromosomes contain our DNA, which tells each cell in our body how to develop and function. Usually, we get 23 chromosomes from each parent, making 46 in total. These 46 chromosomes join into 23 pairs, and these pairs are labeled by numbers 1 through 23.
Instead of having two copies making the typical pair, people with Down syndrome have three copies of chromosome 21. This chromosome difference and the resulting differences in physical and mental development is Down syndrome.
Experts do not know why some people have an extra copy of chromosome 21. There is no evidence that anything in the environment or anything parents do or do not do before or after conception causes Down syndrome. Although mothers older than 35 have a higher chance of having a baby with Down syndrome, it can also happen with younger mothers. There is no known way to prevent Down syndrome.
Characteristics of Down syndrome
People with Down syndrome have some common physical differences. Each person with Down syndrome is unique and may have some or all these features to different degrees. Some physical traits of Down syndrome include:
- Short stature
- Low muscle tone
Upward slanted eyes
- Single deep crease across the palm
- Flattened nasal bridge
Cognitive differences are also present with Down syndrome. According to the National Down Syndrome Society, most people with Down syndrome have mild to moderate cognitive disabilities. Speech delays are also common.
Types of Down syndrome
There are three different types of Down syndrome. Each one results in all or some of the cells having a partial or complete extra copy of chromosome 21:
- Nondisjunction. It is also known as trisomy 21 and is by far the most common type of Down syndrome. About 95% of people with Down syndrome have this type. In nondisjunction, a pair of chromosomes in either the egg or the sperm fail to divide completely just before or at the time of conception. This results in an extra copy of chromosome 21 being passed to the embryo. Every developing and future cell will be affected.
- Mosaicism. It happens when cells in an embryo are dividing, and some cells end up with an extra copy of chromosome 21. Some cells are affected, while others are not. This is rare and only accounts for about 2% of Down syndrome.
- Translocation. Sometimes, an extra piece of chromosome 21 can attach to another chromosome. This is called translocation. Although the result is still a typical 23 pairs of chromosomes, this still causes Down syndrome. About 3% of people with Down syndrome have a translocation.
The CDC estimates that about 7000 people are born with Down syndrome each year in the United States. This is about 1 in every 700 births. Down syndrome is the most common condition resulting from a chromosome difference.
How is Down syndrome diagnosed?
Down syndrome can be diagnosed during pregnancy or after birth. To confirm the diagnosis, a test is needed to examine how many chromosomes are in the baby’s cells. This is called a karyotype.
A doctor may recommend diagnostic tests during pregnancy if prenatal screening estimates a high chance of the fetus having Down syndrome. A procedure to obtain cells from the fetus, such as amniocentesis, chorionic villi sampling, or umbilical blood sampling, is done to obtain fetal cells. Together, parents and their healthcare providers will decide whether to do these tests.
After birth, a healthcare provider may detect the physical characteristics of Down syndrome, even if prenatal screenings were normal. Then, after discussing it with the parents, the doctor, or healthcare provider will order a chromosome blood test for the baby.
Pregnancy screening for Down syndrome
During pregnancy, prenatal screening can be done to look for signs of Down syndrome. Blood tests and ultrasounds are done in the first or second trimester.
Screening tests cannot provide a diagnosis. Prenatal screenings can be falsely positive or negative. However, some screening tests, such as one that looks at fetal cells in the mother’s blood, are highly accurate.
Are there ways to treat Down syndrome?
There is no treatment for Down syndrome. However, there are resources to support people with Down syndrome and their families. Specialists such as speech, occupational, and physical therapists may provide guidance and help optimize development. Special education and social services are also available.
If you are expecting or have a baby who may have Down syndrome, your healthcare providers will guide you. If you have a baby or a loved one with a confirmed diagnosis of Down syndrome and are unsure of the next steps, talking to your healthcare provider and contacting your local Down syndrome society are good places to start.
- National Down Syndrome Society. About Down syndrome.
- CDC. Facts about Down syndrome.
- Genetics in Medicine. Estimation of the number of people with Down syndrome in the United States.
- UpToDate. Patient education: Down syndrome (beyond the basics).
- National Down Syndrome Society. Resources and local support.