Reviewed January 2009
What is trisomy 18?
Trisomy 18, also called Edwards syndrome, is a chromosomal condition associated with severe intellectual disability and abnormalities in many parts of the body. Individuals with trisomy 18 often have a low birth weight; a small, abnormally shaped head; a small jaw and mouth; clenched fists with overlapping fingers; heart defects; and abnormalities of other organs. Due to the presence of several life-threatening medical problems, many infants with trisomy 18 die within their first month. Five percent to 10 percent of children with this condition live past their first year.
How common is trisomy 18?
Trisomy 18 occurs in about 1 in 5,000 newborns. Approximately 80 percent of newborns affected by this disorder are female. Although women of all ages can have a child with trisomy 18, the chance of having a child with this condition increases as a woman gets older.
What are the genetic changes related to trisomy 18?
Most cases of trisomy 18 result from having three copies of chromosome 18 in each cell in the body instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of trisomy 18.
Approximately 5 percent of people with trisomy 18 have an extra copy of chromosome 18 in only some of the body's cells. In these people, the condition is called mosaic trisomy 18. The severity of mosaic trisomy 18 depends on the type and number of cells that have the extra chromosome. The development of individuals with this form of trisomy 18 may range from normal to severely affected.
Very rarely, the long (q) arm of chromosome 18 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in embryonic development. Affected people have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. If only part of the q arm is present in three copies, the physical signs of translocation trisomy 18 may be different from those typically seen in trisomy 18. If the entire q arm is present in three copies, individuals may be as severely affected as if they had three full copies of chromosome 18.
Read more about chromosome 18.
Can trisomy 18 be inherited?
Most cases of trisomy 18 are not inherited, but occur as random events during the formation of eggs and sperm. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 18. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 18 in each of the body's cells.
Mosaic trisomy 18 is also not inherited. It occurs as a random event during cell division early in embryonic development. As a result, some of the body's cells have the usual two copies of chromosome 18, and other cells have three copies of this chromosome.
Translocation trisomy 18 can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 18 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 18. Although they do not have signs of trisomy 18, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
Where can I find information about treatment for trisomy 18?
These resources address the management of trisomy 18 and may include treatment providers.
You might also find information on treatment of trisomy 18 in
Educational resources and Patient support.
Where can I find additional information about trisomy 18?
You may find the following resources about trisomy 18 helpful. These materials are written for the general public.
You may also be interested in these resources, which are designed for healthcare professionals and researchers.
What other names do people use for trisomy 18?
- Complete trisomy 18 syndrome
- Edwards Syndrome
- Trisomy 18 syndrome
What if I still have specific questions about trisomy 18?
Where can I find general information about genetic conditions?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding trisomy 18?
The resources on this site should not be used as a substitute for
professional medical care or advice. Users seeking information about
a personal genetic disease, syndrome, or condition should consult with a qualified
See How can I find a genetics professional in my area? in the Handbook.