health

Almost all cases—92 percent—of Edwards syndrome involve genetic errors
that occur during the formation of germ cells (eggs and sperm) or
during cell division shortly after the egg is fertilized by the sperm.
The most common form of Edwards syndrome occurs when an egg or sperm
carrying two copies of chromosome 18 is involved in conception.
The reason for the extra copy in the abnormal germ cell is a genetic error
called nondisjunction. During the normal process of germ cell formation,
the paired chromosomes in the cell divide so that each daughter
cell has only one member of the pair. In nondisjunction,
one daughter cell gets both members of the chromosome pair and the other
cell has none. If a germ cell carrying two copies of chromosome
18 is fertilized by a normal germ cell from the other parent,
the child will have three copies of chromosome 18. This genetic error is
called a full trisomy 18.
A few cases (about 2 percent) of Edwards syndrome develop when a part of
chromosome 18 becomes attached to another chromosome either before or at
the moment of conception. This type of genetic error is called a translocation.
The child will be born with two copies of chromosome 18 plus some extra genetic
material from chromosome 18 attached to another chromosome. A child with this
type of genetic error is said to have partial trisomy 18.
About 5 percent of cases of Edwards syndrome occur in children
who have some body cells with the extra copy of chromosome 18 and
some body cells without the extra copy. This condition is called mosaic
trisomy 18. It is thought to result from random errors in cell division
during the early stages of fetal development. Children with mosaic
trisomy 18 are less severely affected than those with full trisomy 18 and
have a longer survival period.
The symptoms of Edwards syndrome include a high mortality rate
even before birth. Most embryos with full trisomy 18 die during pregnancy
or are expelled from the mother’s womb in what is called a spontaneous
abortion or miscarriage. Those who survive until birth usually
live only a few days; most die from breathing problems, heart or kidney
defects, generalized infection, or feeding difficulties. Although some children
with mosaic trisomy 18 do not have all the physical features that
characterize the syndrome, the following are considered typical:

• Mental retardation and delayed movement (100 percent of
  infants)
  
• Poor muscle tone, seizures, and breathing difficulties
  
• Abnormally small head, abnormally small eyes, flattened nose,
  small jaw, pointed ears, cleft lip, and cleft palate
  
• Severely retarded growth with clenched hands, missing bone in
  forearm, rocker-bottom feet
  
• Heart abnormalities (90 percent of infants), underdeveloped
  lungs, incomplete esophagus, double spleens, missing appendix,
  and missing gallbladder
  
• Underdeveloped sex organs in both boys and girls
  
• Underdeveloped thyroid and thymus glands