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Inherited Disorders and Birth Defects

Balanced Translocation Embryo Amniocentesis?

01/08/2010

Question:

I have been diagnosed with a balanced translocation of Chrom 14 & 3. My wife and I are 13 weeks pregnant after having 3 mis-carriages. We have just had our 12 week scans. Everything looks to be going well, however our Doctor has advised us there is a chance our child could be born with a mental disability that woudln`t be picked up by an ultra sound, as chromosone 14 would not affect the structural make-up of the bady`s body. To find out if this has occured, they have advised us to take a amniocentesis , which has a miscarrage rate of 1%? I guess I want to know is what are the chances of our baby being born with a birth defect, and should we take the amniocentesis ? Thx.

Answer:

There are 2 things your doctor may be referring to in your case. First, your doctor may be referring to the possibility that, because you have a balanced chromosomal rearrangement, the baby could received an unbalanced rearrangement between chromosome 14 and 3 – a part of either chromosome is missing (deleted) or added (called a duplication). While these pregnancies usually miscarry, there is a chance that the baby could be live born and have mental retardation or other birth defects.

Theoretically, there is 25% chance that the gametes (eggs or sperm) that the mom or dad produces – will have a normal chromosome complement, a 25% chance that the egg or sperm will have the balanced translocation and a 50% chance that the egg or sperm would produce an unbalanced chromosome complement. In your case, the unbalanced complement would be between chromosome # 14 and chromosome #3. However, most of these cases that produce an unbalanced complement will miscarry.

The second problem your doctor may be referring to is that when a balanced translocation involves chromosome # 14 there have been a few cases reported that the baby received 2 chromosome # 14’s (the normal number), but they both came from one parent (called uniparental disomy). Usually, you should get one # 14 from the mother and one from the father. If this happens, which is very rare, the baby could be born with mental retardation and have some other birth defects. These types of problems would not be seen on an ultrasound at 13 weeks.

The only way to tell if there are problems with the chromosomes is to have an amniocentesis done. There is a small risk of miscarriage occurring because of the amniocentesis, however, it is usually less than 1%. The other option is to have a comprehensive ultrasound done about 18-19 weeks of pregnancy. The OB specialist who does these types of ultrasounds will look carefully to see if there are any problems. However, if the ultrasound is normal, there is still a chance that there could be mental retardation or minor birth defects if there is an unbalanced rearrangement or uniparental disomy.

These are important questions. You probably have, but if you have not already spoken to a genetic counselor or geneticist, I would highly recommend that you do. You can locate a genetics center near you at the National Society of Genetic Counselors Resource Center website listed below.

Related Resources:

National Society of Genetic Cousnelors Resource Center

For more information:

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

Response by:

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