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Inherited Disorders and Birth Defects

I Have an Extra 18th Chromosome at 21 Weeks



After having an amnio done, it was discovered that I have an extra 18th chromosome. We had a level 2 ultrasound done to look at specific features of the baby and all revealed normal. Is this possible? If so, why does this chromosome appear if the doctors feel that our child will be born healthy? They told us that one issue has to do with functionality of the brain which cannot be determined until birth. But when looking at the brain they saw nothing to indicate any issues. Same as the heart, face etc. I don`t want to have a pleasant 4 more months but just have some unrest about their findings. Basically, can you have that extra 18th chromosome and still have a 100% healthy child. Thank you for your time.


When an amniocentesis is done, cells from the baby that are floating in the amniotic fluid are collected to look at the chromosomes. If there is an extra chromosome 18, that is called trisomy 18. Babies with trisomy 18 have many problems. In general, babies with trisomy 18 tend to be smaller at birth and can have other features such as the ears being low set, having a small chin, clenching their hands in a particular way, having a short breast bone or a heart defect. All babies born with trisomy 18 are mentally retarded. Because the brain does not function well, most babies with trisomy 18 die within the first few months of life.

When doing a level II ultrasound, often problems can be seen, but not always. However, if a chromosome analysis is done, it is almost 100% accurate. It is not possible to say that the chromosome analysis is totally 100% because a human error can occur - such as samples being mixed up. This is very rare, and good laboratories take extraordinary measures to make sure that things such as this do not happen.

I do not know of any infants born with trisomy 18 that were normal.

I do not know if you have spoken to a genetic counselor, but if you have not, I would highly recommend that you do so. You can locate a genetics center near you at the National Society of Genetic Counselors’ Resource website below.

Related Resources:

National Society of Genetic Counselors Resource Center

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Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University