Infants born with Edward's syndrome 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 Edwards syndrome also have mental retardation, heart defects, and other organ malformations such that most systems of the body are affected.
Edwards syndrome 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 Edwards syndrome increases as a woman gets older.
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 Edwards syndrome. 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 the typical Edwards syndrome.
Trisomy18 is the second most common trisomy and occurs when a baby has three of the eighteenth chromosome.
Mosaic trisomies occur when the extra chromosome is present in some (not all) of the cells, and partial trisomies occur when only part of an extra chromosome is present.
See How Trisomy18 is Diagnosed for a full explanation of these tests and which are screening tests and which are diagnostic.