Trisomy 21 (Down Syndrome) | Prenatal Screening Ontario

What is trisomy 21?

Trisomy 21 (also called Down syndrome) is caused by an extra chromosome 21. Babies with trisomy 21 have three copies of this chromosome, instead of the typical two.

How does trisomy 21 happen?

The extra chromosome 21 generally happens by chance in the sperm or the egg that makes the baby. Trisomy 21 does not generally run in families.

What does trisomy 21 mean for a baby?

The impact of the extra chromosome 21 varies a lot from person to person. Everyone is unique, with their own strengths and challenges:

Some pregnancies with trisomy 21 result in miscarriage or later pregnancy loss (stillbirth).
Children with trisomy 21 have some facial features and other physical aspects in common. They also look a lot like their parents.
All people with trisomy 21 have some degree of intellectual disability. This means children with trisomy 21 often take longer to learn. As they get older, people with trisomy 21 need more support with personal care and daily tasks. The level of support that someone needs varies from person to person.
Certain health issues happen more often for people with trisomy 21 than those who do not have this genetic condition. Some people with trisomy 21 do not have any of these issues, while others have several. Possible health concerns include:
- heart defects (present at birth)
- eyesight and hearing difficulties
- feeding and digestion difficulties
- respiratory infections
- blood disorders
- sleep issues
- Alzheimer’s disease later in life (a condition that affects memory, thinking and behavior)
There is treatment and special support to help people with trisomy 21, but there is no cure.
People with trisomy 21 usually live into their 60s and are now able to be more independent than ever before. Many adults lead fulfilling lives and can take part in school, work and their communities.

How often does trisomy 21 happen?

About 1 in every 800 babies in Canada are born with trisomy 21. Any person can have a baby with trisomy 21, even if no one in their family has this condition. The chance for this to happen increases as the pregnant person gets older.

How can trisomy 21 be detected during pregnancy?

Sometimes there are clues during the pregnancy that tell us the baby might have a chromosome difference, like trisomy 21. Diagnostic testing can then be done so that you know for sure whether the baby has a chromosome difference.

| Prenatal screening tests

All pregnant people in Ontario have the option of prenatal genetic screening. This testing will tell you the chance for having a baby with trisomy 21. Your prenatal screening options depend on how far along you are in the pregnancy, the tests available in your area and how many babies you are expecting.

| 18-22 week (detailed anatomy) ultrasound

Some babies with trisomy 21 will show certain signs on the 18-22 ultrasound. It is important to know that ultrasound has its limitations. Some babies with trisomy 21 do not show any signs on ultrasound at all.

Birth defects

Babies with trisomy 21 might have birth defects detected on ultrasound, such as:

- - heart defect and
  - narrowing or blockage of the intestine (duodenal atresia)

Soft markers

Some babies with trisomy 21 will have soft markers detected on ultrasound. Soft markers are not "birth defects". Soft markers are signs that the baby could have a chromosome difference.

Birth defects and soft markers can be caused by many different conditions, not just trisomy 21. You should be offered a referral for genetic counselling to discuss your results and options for more testing.

| Diagnostic testing

The only way to know for sure if the baby has trisomy 21 is through a diagnostic test. Diagnostic testing is not generally done during the pregnancy unless there are signs that the baby could have a genetic condition. This testing can be done by looking at cells from the placenta (chorionic villus sampling) or from the fluid around the baby (amniocentesis).

Some people do not wish to have diagnostic testing during the pregnancy. In that case, diagnostic testing can be done after birth by getting some blood from the umbilical cord, or through a regular blood draw from the baby.

Limitations to prenatal testing

Some health concerns can be seen through ultrasound before the baby is born. However, it is not possible to know all the health challenges and abilities a child with trisomy 21 will have. In the same way, we cannot predict how a family will cope with raising a child with this condition.

It is important that families are offered referrals to health-care practitioners with up-to-date knowledge and experience with trisomy 21, and who are able to consider families' values and beliefs.

Types of trisomy 21

There are different types of trisomy 21 that can happen:

Trisomy 21

It is thought that 95% of cases of trisomy 21 are the result of having three copies of chromosome 21 in every cell of the body. The extra chromosome 21 is not attached to any other chromosomes. This type of trisomy 21 generally does not run in a family.

Mosaic trisomy 21

A smaller number of individuals with trisomy 21 have an extra chromosome 21 in only some of the body's cells. These individuals have a mixture of cells: some have the typical two copies, and others have three copies. The impact on a baby's development varies widely and depends on the number of cells that have the extra chromosome 21. People with the mosaic form often have milder symptoms. Mosaic trisomy 21 happens randomly early in pregnancy, and it is not passed down from other family members.

Translocation

In individuals with a translocation form of trisomy 21, part of chromosome 21 becomes attached (translocated) to another chromosome. These individuals have two separate copies of chromosome 21 plus an extra chromosome 21 attached to another chromosome. This results in three copies of chromosome 21. This type of trisomy 21 is less common and can sometimes be passed down from a healthy parent.

More information

You can get more information from other medical and Down syndrome organizations. These can be helpful resources when looking for a range of views on living with this chromosome difference and when you are looking for information in other languages.

Sick Kids - Down Syndrome (English, French, Arabic, Chinese Simplified, Chinese Traditional, Spanish, Tamil, Urdu)
Mayo Clinic - Down Syndrome (English, Spanish, Arabic, Chinese)
The Down Syndrome Association of Ontario
Canadian Down Syndrome Society
The Down Syndrome Association of Toronto
Down Syndrome Pregnancy
Lettercase - Understanding Down Syndrome
National Down Syndrome Society
National Down Syndrome Congress
Lumind Down Syndrome Foundation
MedlinePlus
Up to Date - Patient Education: Down Syndrome