Trisomy 18

What is Trisomy 18?

Trisomy 18 (also called Edwards syndrome), is a chromosome difference that is caused by an extra copy of chromosome 18. Babies with trisomy 18 have three copies of this chromosome, instead of the typical two. This generally happens by chance in the sperm or the egg that made the baby, and does not typically run in families.

What Does Trisomy 18 Mean For a Baby?

Having an extra chromosome 18 disrupts the typical development of the baby in the womb, resulting in a severe, life-limiting condition, affecting many different parts of the body.

Many pregnancies with trisomy 18 do not make it to birth, and result in miscarriage or stillbirth. Babies with trisomy 18 who are born and survive the birth usually have severe intellectual disability and health challenges. Babies can go home to their families, but require substantial caregiving.

A small percentage of babies with trisomy 18 can live past the first year of life, and some individuals have survived to their teenage years and beyond.

Chromosomes in the cell of a person with trisomy 18

How Often Does Trisomy 18 Happen?

About 1 in 6,000 to 1 in 8,000 babies are born with trisomy 18. Although trisomy 18 can happen at any age, the chance increases with the age of the pregnant individual. Trisomy 18 is more common than that during pregnancy, but many babies with this chromosome difference do not make it to birth.

Can Trisomy 18 Be Detected During Pregnancy?

Prenatal Screening Tests

All pregnant individuals in Ontario have the option of prenatal screening which includes screening for trisomy 18. The following prenatal screening options may be available depending on how far along you are in the pregnancy or availability in your area:

You may be hearing about trisomy 18 because the results of one of your prenatal screening tests was "positive" or "high risk" for having a baby with trisomy 18. If this is the case, your health-care provider should discuss your options which may include a referral for genetic counselling.

Ultrasound

The extra chromosome 18 can cause a wide range of problems (sometimes referred to as birth defects) in the developing baby, and these may be seen on one of the ultrasounds that you have in pregnancy. Most babies with trisomy 18 will have concerns on the detailed anatomy ultrasound, which can 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)
cleft lip or palate - incomplete closure of the lip or roof of the mouth during development
differences in the structure of the kidneys (e.g. a missing kidney)

Birth defects can be caused by many different conditions, not just trisomy 18. If anything significant is seen on ultrasound, you should be offered a referral for genetic counselling to discuss your results further and options for more testing.

Diagnostic Testing

Although the above tests can increase the chance that the baby has trisomy 18, the only way to know for sure is through a diagnostic test. This testing can be during pregnancy (through chorionic villus sampling or amniocentesis), or after birth.

Types of Trisomy 18

Trisomy 18

The majority of cases of trisomy 18 result from an individual having three copies of chromosome 18 in each cell in the body instead of the usual two copies. This form of trisomy 18 typically happens by chance and is not passed down in the family.

Mosaic trisomy 18

Rarely, some individuals with trisomy 18 have an extra copy of chromosome 18 in only some of the body's cells. These individuals have a mixture of cells, some having the typical two copies of chromosome 18, and some having three copies. The impact of having this extra genetic material on a baby's development varies widely and depends on the number of cells that have the extra chromosome 18. Mosaic trisomy 18 is not inherited (passed down in the family).

Partial trisomy 18

Partial trisomy 18 is very rare condition that happens when an individual 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 is dependent on the amount of extra genetic material present from chromosome 18. This type of trisomy 18 can sometimes be passed down from a healthy parent.

Additional Resources

MedlinePlus
SOFT – Support Organization for Trisomy 13, 18 and related disorders
Trisomy 18 Foundation
International Trisomy 13/18 Alliance
The Chromosome 18 Registry & Research Society
Rare Disease Database - Trisomy 18