How to Conquer a Chromosome Abnormality—
Why and how did this happen?
University of Texas Health Science Center at San Antonio, School of Medicine
The Chromosome 18 Clinical Research Center
Among the top questions
that parents have are:
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How common are chromosome abnormalities?

Why did this happen to me?

When did this happen?

Did I do something that
caused this?
Inside this issue:
Different Chromosome
Abnormality Proportions
2
What did I do?
2
Summary
2
Why and how did this happen?
The fact is that chromosome abnormalities occur very commonly. Chromosome rearrangements, deletions
and/or duplications can happen during
cell division when chromosomes replicate and then segregate into two
daughter cells. Additionally, the separation of chromosomes into two
daughter cells can have errors in
which one cell gets one too many
chromosomes leaving the other cell
with too few chromosomes. These
errors are believed to happen during
the formation of the sperm or egg
cells, before conception, or very soon
after fertilization.
These types of errors are believed to
occur in 50% of conceptions. However, when these errors happen, 50% of
the time the resulting embryo fails to
implant in the uterus, too early to even
become a recognized pregnancy. Of
those abnormal embryos that do implant, 90% are miscarried, usually
during the first trimester.
Incidence of Chromo‐
some Abnormali es If you do the math, 50% of conceptions have chromosome abnormalities,
but 90% of those do not implant. That
leaves 5% of conceptions with chromosome abnormalities surviving long
enough to become a recognized pregnancy. But if 90% of those are miscarried, that leaves 1/2 of 1% of conceptions having chromosome abnormalities that are survivable to birth.
This is the same as one out of every
200 babies having a chromosome
abnormality.
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50% of concep ons 90% do not implant 90% spontaneously aborted 0.5% in newborns These data are derived from what is
known about in vitro fertilization.
Incidence of chromosome abnormality in live births
The actual incidence in live births is
slightly higher—one in every 180. In a
population of individuals with intellectual disability, 50% of them have
chromosome abnormalities.
The caveat is that this data was derived before the use of the high resolution molecular analysis techniques
used today. Therefore, we predict that
chromosome abnormalities are more
common because we can now detect
small abnormalities that were once
undetectable. We also predict that
chromosome abnormalities are the
cause of an even higher percentage of
the cases of intellectual and developmental disability.
There will be many individuals with
chromosome abnormalities who have
no manifestation as a consequence.
Frequency of Chromosome
Abnormalities in:
*Live births—1 in 180
*Intellectual Disability—50%
How to Conquer a Chromosome Abnormality—Why and how did this happen?
Page 2
Different Chromosome Abnormality Proportions
This pie chart illustrates the proportions of some different chromosome
abnormalities. Down syndrome is, by
far, the most common, with Trisomy 18
and Trisomy 13 a distant 2nd and 3rd.
All other possible combinations make
up 54% of all chromosome abnormalities. Individually, these conditions are
rare—some being unique to a single
person, but collectively this represents a large number of persons.
Down
Trisomy 18
Trisomy 13
All other
Data from March of Dimes website 2007.
Did I do anything or was I exposed to anything that caused this to happen?
By now you know that chromosome
abnormalities happen as a part of life
and cell division—a process that does
not always occur perfectly.
“Probably not!”
But the question remains, did you do
anything or were you exposed to
anything that increased your chances
that this would happen to you? The
answer is probably not. There is no
data to suggest that any environmental exposures before or during preg-
nancy can actually increase your
chances of having a child with a chromosome abnormality.
It is known that older mothers have an
increased risk for having a child with
an extra chromosome, such as Down
syndrome. It is also known that older
fathers are at risk for having a child
with a new mutation, but not necessarily a chromosome abnormality.
One might think that the survivors of
Summary
So to summarize what we have
learned so far:
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Chromosome abnormalities
occur in one of every 180 live
births.
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They happen to everyone—but
most are not compatible with
life.

The chromosome abnormality
occurred in the germ cells probably before conception or very
early after fertilization.

There is currently no data that
suggests this could have been
prevented in parents with normal chromosomes.
the Hiroshima nuclear bomb and their
descendants might have had more
children with chromosome abnormalities die due to increased radiation
exposure—but they have not.
This leads us to the conclusion that
for parents who themselves have
normal chromosomes, there is no way
to prevent having a child with a chromosome abnormality except to have
none at all.
UTHSCSA—Department of Pediatrics
The Chromosome 18 Clinical Research Center
MSC 7820
7703 Floyd Curl Drive
San Antonio, TX 78229-3900
Phone: 210-567-5321
Fax: 210-567-0919
E-mail: [email protected]
For more information, you may contact the authors and principal investigators of the Chromosome 18 Clinical
Research Center at the phone numbers or email shown to the left.
Authors & Principal Investigators:
Jannine D. Cody, PhD and Daniel E. Hale, MD
Our Motto
To provide individuals and families affected by
chromosome 18 abnormalities with comprehensive
medical and educational information with a focus on
treatment options.
We are on the web!
http://pediatrics.uthscsa.edu/centers/chromosome18/
Information provided by The Chromosome 18 Clinical
Research Center to:
http://www.chromosome18.org/
210-657-4968