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Wednesday, November 26, 2014
Inherited Disorders and Birth Defects
Miscarriages From Balanced Translocation
I have been recently diagnosed with a balanced translocation on chromosomes 8 and 20. We have one living daughter with no problems and have had three miscarriages all prior to 8 weeks. All of these miscarriages have been blighted ovums. I am now 8 weeks pregnant and have had an ultrasound and the baby does have a heartbeat. I am wondering what my chances are of this baby having an unbalanced translocation and what effect would this have on my child?
A translocation occurs when two pieces of chromosomes break off and switch places with each other. If all the chromosomal material is present, just rearranged (such as the translocation that has been diagnosed in you - a rearrangement between chromosomes 8 and 20) – this person is called a balanced translocation carrier and should have no health problems since all the chromosomal material needed is present and functioning properly.
However, when a person with a balanced chromosomal rearrangement forms eggs or sperm, some of the chromosomal material can be lost or duplicated so that the developing embryo has too much or not enough genetic material. This is what leads to an unbalanced translocation and usually a miscarriage.
In theory, there is a 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 # 8 and chromosome # 20. In all of these cases that produce an unbalanced complement - there is a very high probability (greater than 99%) that the pregnancy will miscarry. However, depending on where the switch between the chromosomes happened, there could be a small chance that a pregnancy with an unbalanced translocation would continue. In this case, the baby could have birth defects and/or developmental problems.
The specific types of problems or birth defects would depend on the specific areas of the chromosomes that were lost or duplicated in the chromosomes that are translocated and what specific genes are located at these sites. For many unbalanced rearrangements (translocations) it is not possible to predict what abnormalities to expect; for others the medical literature may provide information.
The chance of having a miscarriage if someone has a translocation can vary a great deal and depends on the specific type of rearrangement involved. However, as you know because of your daughter, they can have healthy pregnancies.
You ask excellent questions. If you haven’t already done so, I would highly recommend that you sit down with a genetic counselor or geneticist and talk about these risk figures specifically in regard to your chromosome rearrangement. The National Society of Genetic Counselors resource link can help you locate a genetics center near you.
Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University