Trisomy 18 or Edwards Syndrome (named after John H. Edwards) is a genetic disorder. It is the second most common trisomy after Down's Syndrome. It is caused by the presence of three - instead of two - chromosomes 18 in a fetus or baby's cells. more...
The additional chromosome usually occurs before conception, when egg and sperm cells are made. A healthy egg or sperm cell contains 23 individual chromosomes - one to contribute to each of the 23 pairs of chromosomes needed to form a normal cell with 46 chromosomes. Numerical errors arise at either of the two meiotic divisions and cause the failure of segregation of a chromosome into the daughter cells (non-disjunction). This results in an extra chromosome making the haploid number 24 rather than 23. Fertilization of these eggs or sperm that contain an extra chromosome results in trisomy, or three copies of a chromosome rather than two. It is this extra genetic information that causes all the abnormalities characteristic of individuals with Edwards Syndrome. As each and every cell in their body contains extra information, the ability to grow and develop appropriately is delayed or impaired. This results in characteristic physical abnormalities such as low birth weight; a small, abnormally shaped head; small jaw; small mouth; low-set ears; and clenched fists with overlapping fingers. Babies with Edwards syndrome also have heart defects, and other organ malformations such that most systems of the body are affected.
Edwards Syndrome also results in significant developmental delays. For this reason a full-term Edwards syndrome baby may well exhibit the breathing and feeding difficulties of a premature baby. Given the assistance offered to premature babies, some of these infants are able to overcome these initial difficulties, but eventually succumb.
The survival rate for Edwards Syndrome is very low. About half die in utero. Of liveborn infants, only 50% live to 2 months, and only 5 - 10% will survive their first year of life. Major causes of death include apnea and heart abnormalities. It is impossible to predict the exact prognosis of an Edwards Syndrome child during pregnancy or the neonatal period. As major medical interventions are routinely withheld from these children, it is also difficult to determine what the survival rate or prognosis would be for the condition if they were treated with the same aggressiveness as their genetically normal peers. They are typically severely to profoundly developmentally delayed.
The rate of occurrence for Edwards Syndrome is ~ 1:3000 conceptions and 1:6000 livebirths, as 50% of those diagnosed prenatally with the condition will not survive the prenatal period. Although there is an increased risk of conceiving a child with Edwards Syndrome as a woman's age increases, women in their 20's and 30's still conceive Edwards Syndrome babies.
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 the abnormalities are often less than for the typical Edwards syndrome.
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