Most of the time, trisomy 18 happens because a baby has three copies of chromosome 18 in each cell of the body (instead of the usual two copies). This form of trisomy 18 generally happens by chance and is not passed down in the family.
Trisomy 18 (Edwards Syndrome)
What is trisomy 18?
Trisomy 18 (also called Edwards syndrome) is caused by an extra copy of chromosome 18. Babies with trisomy 18 have three copies of this chromosome, instead of the typical two.
How does trisomy 18 happen?
The extra chromosome 18 generally happens by chance in the sperm or the egg that makes the baby. Trisomy 18 does not generally run in families.
Illustration adapted from Genetic Counseling Aids, 7th Edition, Copyright 2020, permission for use granted by Greenwood Genetic Center
What does trisomy 18 mean for a baby?
Having an extra chromosome 18 affects how different parts of the body grow and form. Many babies with trisomy 18 do not survive to the end of the pregnancy. Pregnancies with trisomy 18 often end up in miscarriage or stillbirth (pregnancy loss later in the pregnancy). If born alive, babies with trisomy 18 usually have severe intellectual disability and health problems. Babies can go home to their families, but require a lot of caregiving. A small number of babies with trisomy 18 can live past the first year of life, and some individuals have survived to their teenage years and beyond.
How often does trisomy 18 happen?
About 1 in every 6000 to 8000 babies is born with trisomy 18. Although trisomy 18 can happen at any age, the chance increases as the pregnant person gets older.
How can trisomy 18 be detected during pregnancy?
There can be signs for trisomy 18 during the pregnancy. Diagnostic testing can then be done so that you know for sure whether or not the baby has trisomy 18.
| Prenatal screening tests
All pregnant people in Ontario have the option of prenatal genetic screening. This testing will tell you the chance to have a baby with trisomy 18. Your prenatal screening options depend on how far along you are in the pregnancy, tests available in your area and how many babies you are expecting.
| Ultrasound
The extra chromosome 18 can cause a wide range of birth defects. Most babies with trisomy 18 will have concerns seen on the 18-22 week (detailed anatomy) ultrasound, and sometimes even on an earlier ultrasound. These include:
- Smaller size than expected
- Differences in the structure of the brain
- Clenched hands with overlapping fingers
- Heart defect
- Small head (microcephaly)
- Small jaw (micrognathia)
- Incomplete closure of the lip or roof of the mouth during development (cleft lip or palate)
- Kidney problem (e.g. a missing kidney)
Birth defects and problems with the growth of the baby can be caused by many different conditions, not just trisomy 18. You should be offered a referral for genetic counselling to discuss your results and options for more testing.
| Diagnostic testing
Although the above tests can tell you there is an increased chance for the baby to have trisomy 18, the only way to know for sure is through a diagnostic test. 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. Diagnostic testing can be done after birth by getting a bit of blood from the umbilical cord, or through a regular blood draw from the baby.
Rarely, some babies with trisomy 18 have an extra copy of chromosome 18 in only some of the body's cells. These babies have a mixture of cells: some cells have the typical two copies, and other cells have three copies. The impact of having this extra genetic material on a baby's development varies widely. The impact depends on the number of cells that have the extra chromosome 18. Mosaic trisomy 18 happens by chance and is not passed down in the family.
Partial trisomy 18 is very rare. It happens when a baby has an extra part of chromosome 18, which can often be attached to another chromosome (this is referred to as a translocation). The impact on development in individuals with partial trisomy 18 varies and depends on the amount of extra genetic material. This type of trisomy 18 can sometimes be passed down from a healthy parent.
More information
You can get more information from other medical and Trisomy 18 organizations. These can be helpful resources when looking for a range of views on having this chromosome difference.
- MedlinePlus
- SOFT – Organisation offrant du support pour les trisomies 13, 18 et autre conditions connexes
- Trisomy 18 Foundation
- International Trisomy 13/18 Alliance
- The Chromosome 18 Registry & Research Society
- Rare Disease Database - Trisomy 18
Contact Us
Prenatal Screening Ontario
CHEO Research Institute
Centre for Practice-Changing Research Building
401 Smyth Road
Ottawa, ON K1H 8L1
Information line
Phone: 613-737-2281
Toll-Free: 1-833-351-6490
Monday to Friday, 9:00 AM to 3:00 PM EST
Subscribe to PSO Updates
Subscribe to stay informed with the latest PSO news and prenatal screening updates in Ontario.