Trisomy 18

Trisomy refers to a person having three copies of a chromosome, instead of the typical two. Trisomy 18 (also called Edwards syndrome), is a condition that is caused by an extra copy of chromosome 18. This generally occurs in the sperm or the egg that conceived the baby.
The extra copy of chromosome 18 is usually found in every cell of a person's body.
When a person has an extra chromosome 18, it disrupts the typical pattern of development resulting in a severe, life-limiting condition, affecting many different parts of the body.

trisomy 18 karyotype

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 is typically not inherited.

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 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. Like Trisomy 18, mosaic trisomy 18 is not inherited.

Partial trisomy 18

Partial trisomy 18 is very rare condition that occurs when an individual has an extra part of chromosome 18, which can often be attached to another chromosome (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 inherited from a healthy (or unaffected?) parent.

The Incidence of Trisomy 18

Trisomy 18 occurs in 1 in 6,000 to 1 in 8,000 live births and 1 in 2500 to 1 in 2600 pregnancies. The overall prevalence is higher in pregnancy because many babies with trisomy 18 will miscarry before delivery. Although a pregnant person could have a baby with trisomy 18 at any age, the chance increases with maternal age.

Features of Trisomy 18

The extra chromosome 18 can affect the development of each baby differently. In general, babies with trisomy 18 have severe intellectual disabilities and can have birth defects in almost every organ system in their body. Some common features include:

Low birth weight
Pocket of fluid in the brain (choroid plexus cyst(s))

Clenched hands
Heart defects

Kidney abnormalities
Spina bifida (spine not fully closed)

Cleft lip and/or palate (incomplete closure of the lip or roof of the mouth during development)
Club foot
Rocker bottom feet (feet appear shaped like the rocker of a rocking chair)

Eye and ear problems (e.g. cataracts, unusually small eyes)
Microcephaly (small head)
Micrognathia (small chin)

Life Expectancy

The life expectancy of babies with trisomy 18 is difficult to determine, however, it is estimated that 50% of babies with trisomy 18 survive past the first week of life, and 5-10% of children live beyond the first year.

Next Steps

You may be hearing about trisomy 18 because your prenatal screening test (eFTS, MSS, NIPT) was positive or high risk for having a baby with trisomy 18. If this is the case, your health care provider should offer to refer you for genetic counselling to review your test results and your options for additional testing.

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

Sources for This Page

Cereda A, Carey JC. The trisomy 18 syndrome. Orphanet J Rare Dis. 2012;7:81.

U.S. National Library of Medicine. Genetics home reference [Internet]. Bethesda (MD): U.S. National Library of Medicine; 2017.