Inherited Disorders and Birth Defects

Gnetic birth defect due to tresame 18 chromos

06/04/2002

Question:

i want to know the genectic affect of the 18th chromosome. i think it is called tresame 18. my daughter had an amnio done today. she is about 20 weeks pregnant. they found some fluid in the brain. the test results won`t be back for 14 days. is this condtion called hydoencephalis. my mother had a child, her fourth born with water on the brain that didn`t survive. could this be geneticlly transferred to my daughter. thanks

Answer:

The disorder you are describing is Trisomy 18 syndrome - where there are 3 number 18 chromosomes instead of the normal number of 2. These infants usually have multiple birth defects including being small at birth, having abnormal ears, may have heart defects or kidney defects, other brain abnormalities and all have mental retardation. In a small number of cases they can also have hydrocephalus (water on the brain). Due to the multiple problems these babies have, they usually do very poorly at birth and most of them die within the first few weeks or months of life.
From your description, it appears that your daughter`s doctors feel that the baby may have Trisomy 18. The results from the amniocentesis should be able to confirm the doctors` suspicions. Cells from the baby are floating in the amniotic fluid. It is these cells that the laboratory will grow in culture and then examine to look at the baby`s chromosomes. Most of the time when the diagnosis is trisomy 18, it is due to an extra number 18. There is no specific cause known for why eggs or sperm do not divide correctly. But if that occurs, then it can lead to the baby having an extra chromosome.
In a very small percentage of couples, one of the parents might have a rearrangement of their chromosomes - where two of their chromosomes are attached to one another. This is called a balanced translocation. As long as all the genetic material is present and functioning properly, there is no effect - the person would not know they had such a rearrangement. However, when the egg or sperm carrying this balanced rearrangement is fertilized at conception, the baby can receive too much or not enough genetic material. This is called an unbalanced chromosomal rearrangement. When chromosome 18 is involved, it can lead to the baby having trisomy 18. When the results of the amniocentesis come back, your daughter will know if this is what happened in her case.
Hydrocephalus is a common birth defect. The fact that your mother had a baby with hydrocephalus may or may not be related to the problems with your daughter. I would recommend that your daughter talk to a genetic counselor or geneticist to discuss these issues in detail if she has not already done so.
There is an excellent website, The Chromosome 18 Registry and Research Society, which is listed below that has information about trisomy 18. Also, your daughter can ask her doctor for a referral to a genetics center or she can find a genetics center near her at the NSGC Resource website also listed below.

Related Resources:

Chromosome 18 Registry and Research Society
National Society of Genetic Counselors Resource Link

For more information:

Go to the Inherited Disorders and Birth Defects health topic, where you can:

Read articles on this topic
Browse the previously asked questions
Ask your own question.

Response by:

Anne Matthews, RN, PhD
Director, Genetic Counseling and Family Studies
Associate Professor
Department of Genetics
School of Medicine
Case Western Reserve University