Trisomy 18 (Edwards Syndrome)

What is trisomy 18?

Trisomy 18, also called Edwards syndrome, is a chromosomal condition that occurs when there are three copies of chromosome 18 in each cell instead of the normal two copies. Infants born with trisomy 18 have an extremely high mortality rate—only 5 to 10 percent of these infants survive the first year of life. Some characteristics of this condition are low birth weight; a small, abnormally shaped head; small jaw; small mouth; and clenched fists with overlapping fingers. Infants born with trisomy 18 also have mental retardation, heart defects, and other organ malformations such that most systems of the body are affected.

How common is trisomy 18?

Trisomy 18 affects about 1 in 5,000 to 6,000 live births. Approximately 80 percent of cases occur in females. The risk of having a child with trisomy 18 increases as a woman gets older.

What are the genetic changes related to trisomy 18?

Trisomy 18 is a chromosomal condition related to chromosome 18.

People with trisomy 18 have additional DNA from chromosome 18 in some or all of their cells. The extra material disrupts the normal course of development, causing the characteristic features of trisomy 18.

Trisomy 18 results when each cell in the body has three copies of chromosome 18 instead of the usual two copies. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic trisomy 18.

Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. Affected people have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 18 and often the physical signs of the syndrome differ from those typically seen in trisomy 18.

Can trisomy 18 be inherited?

Most cases of trisomy 18 are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction can result in reproductive cells 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 error during cell division early in fetal 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 the chromosome.

Trisomy 18 due to a translocation is extremely rare, but may 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.

What other names do people use for trisomy 18?

• Chromosomal imbalance syndrome, pair 18, trisomy
• Complete trisomy 18 syndrome
• E3 Trisomy
• Edwards Syndrome
• Trisomy 16-18
• Trisomy 18 syndrome
• Trisomy E syndrome