Last Reviewed June 2, 2009
Symptoms & Characteristics
Trisomy 18 (also known as Edwards syndrome) is associated with mental retardation and physical abnormalities. Some common physical abnormalities can include:
- small head (microcephaly) that is abnormally shaped
- clenched fists with overlapping fingers
- kidney defects and problems
- underdeveloped fingernails
- undescended testicle (cryptorchidism) in males
- unusual shaped chest (pectus carinatum)
The majority of pregnancies affected with Trisomy 18 will spontaneously miscarry during the pregnancy. For the pregnancies that do survive until birth, there is an extremely high mortality rate in infancy. About 5% to 10% of infants with Trisomy 18 survive their first year of life.
Treatment
There is currently no cure for Trisomy 18. Treatment involves managing the symptoms.
How Common Is It?
Trisomy 18 affects about 1 in 5,000 to 1 in 6,000 newborns worldwide.
Genetics & Inheritance
All of our genes are packaged together on chromosomes. People typically have 46
chromosomes in total, which are grouped into 23 pairs. The first 22 pairs
are numbered largest to smallest (1-22) and are called autosomes. The
last pair are called the sex chromosomes and determine gender, either XX
(female) or XY (male). We typically inherit 23 chromosomes (one of each pair)
from our mother (in an egg cell) and the other 23 from father (in a sperm
cell).
Trisomy 18 is caused by the presence of an extra copy of chromosome 18 in every cell of a person's body.
In most cases, the extra copy is caused by a random "non-disjunction event" in either the egg or the sperm cell.
- Non-disjunction occurs when chromosomes in the developing egg or sperm fail to separate during cell division. The result is a sperm or egg cell with either an additional chromosome or one that lacks a chromosome. When this sperm or egg cell combines with one from the opposite sex, the resulting embryo will have cells with an extra chromosome (47) or cells that have a missing chromosome (45).
- The chance for a "non-disjunction event" to occur in an egg increases with age of the biological mother (maternal age). As a consequence, so does the risk for Trisomy 18 and other chromosome trisomies.
In some cases, the extra copy is caused by a chromosome translocation.
- This type of rearrangement can be inherited from a parent. 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 or missing genetic 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 to have children with Trisomy 18. People who carry this type of balanced translocation may also be at increased risk to have pregnancy losses, stillbirths, and children with other chromosome abnormalities.
- Parents and family members of an affected person may want to consider chromosome analyses themselves. This testing should be discussed with a qualified health care professional.
In a few cases, the extra copy is caused by a random error during cell division early in fetal development (after conception has already occurred). As a result, only some (not all) of the body's cells have the usual two copies of chromosome 18, and other cells have three copies of chromosome 18. This is called Mosaic Trisomy 18.
In rare cases, only part of chromosome 18 will be duplicated. This will lead to extra copies of some, but not all, of the genes on chromosome 18.
There are other genetic conditions that have symptoms that overlap with Trisomy 18. A genetic consultation with a trained genetic professional is important for a complete evaluation, accurate diagnosis, as well as discussion of the benefits and limitations of chromosome testing and recurrence risk.
Genetic Testing
Trisomy 18 can be diagnosed by a
chromosome analysis, which is available through an
in person
genetic consultation for people who are considered at risk. Use
our
find a genetic professional directory to locate a trained genetic professional in your area.
Support & More Information
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Alternative Names
- Edwards syndrome
- Chromosomal imbalance syndrome, pair 18, trisomy
- Complete trisomy 18 syndrome
- E3 Trisomy
- Trisomy 18 syndrome
- Trisomy E syndrome
- 47, XX + 18
- 47, XY + 18