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WHAT IS TRISOMY 18?
At conception, 23 chromosomes from the father and 23 chromosomes from the mother combine to create a baby with 46 chromosomes in each cell, two of
each type. Each chromosome holds a particular "position" in the cell and
carries certain genetic material. A trisomy occurs when a baby has three chromosomes in one position instead of the normal two.
The most common trisomy is Trisomy 21, also known as Down Syndrome,
where a baby has three of the twenty-first chromosome. Trisomy 18 is the
second most common trisomy and occurs when a baby has three of the
eighteenth chromosome. This results in 47 chromosomes instead of the
normal 46 in the affected cells. It is this extra genetic material that causes
the problems associated with Trisomy 18.
Trisomy 18 is also called Edwards syndrome and occurs in about 1:3000 live
births. Unlike Down syndrome, Trisomy 18 is usually fatal, with most of the
babies dying before birth and those who do make it to birth typically living
only a few days. However, a small number of babies (<10%) live at least one
Most trisomies (about 95%) are full trisomies. That is, the extra chromosome
occurs in every cell in the baby's body. This type of trisomy is not hereditary, and is not due to anything the parents did or did not do, and it is by far the most common type. In most literature, Trisomy 18 means full Trisomy 18.
Another type of trisomy is Trisomy 13 (Patau's Syndrome), which occurs less frequently than Trisomy 18. Although full trisomies may occur at any position,
the extra genetic material usually causes such problems that the baby cannot
make it to term. As a result, these three trisomies: 21, 18, and 13, are the
only full trisomies that have ever led to a live birth.
There are also partial and mosaic trisomies, which are much more rare.
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. Some partial trisomies are translocations, usually
caused by a parent's translocation, which may be hereditary.
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