I have a balanced translocation of chromosomes 6 and 14. I found this out after my brother's baby died because of unbalanced translocation. I understand this problem occurs at conception. My daughter, now 9, also has a balanced translocation. Do you have any information on this condition? What are my chances that a future pregnancy would involve an unbalanced translocation? What effects would that have on the baby? Does the risk increase with age? I expect I will be about 35 when I try to have another baby. I understand that pregnancy at that age carries certain risks, but I can't find information on the 6/14 translocation.

J.W.

A chromosomal translocation is a condition in which a small piece of one chromosome switches places with a small piece of another chromosome. In your case, some of what belongs on chromosome number 6 is found on number 14, and some of what should be on number 14 is found on number 6. You have all the necessary chromosomal material. It just resides on a different chromosome.

This condition is not like having an extra or missing chromosome. Fetuses with extra or missing chromosomes often end in miscarriage, although some cases of Down syndrome (trisomy 21) or Turner syndrome (45X) are born alive. In contrast, the carrier of a balanced translocation appears -- and for almost all purposes is -- normal, since there is no missing or extra chromosomal material. The problem seems to occur when trying to conceive.

We all have 23 pairs of chromosomes for a total of 46. In each pair, one of the chromosomes comes from each parent. When it is our turn to make sperm or eggs, we need to get rid of one chromosome from each pair so the sperm or egg contributes only 23 chromosomes to the future pregnancy. The outcome of the pregnancy depends on which chromosome each particular egg or sperm end up. So you have one normal chromosome 6 and a normal 14; you also have one copy of each that contains the translocated information from the other chromosome. If your egg gets the chromosomes 6 and 14 that are normal, your offspring will be normal. If the egg gets both chromosomes that are translocated, the baby will be normal, but will carry the same chromosomal pattern as you. But if the egg gets a good chromosome 6 and a translocated 14, then there will be an extra portion of chromosome 6 and a missing portion of chromosome 14. This is called an unbalanced translocation. The same problem occurs if your egg gets a normal 14 and a translocated 6. In either of these cases, the embryo gets too much of one chromosome and not enough of the other.

Statistics would suggest that 50 percent of the eggs would likely generate unbalanced translocations, 25 percent would be balanced translocations (like you) and 25 percent would be normal. In practice, many pregnancies that result from chromosomal imbalances miscarry before the woman is even aware that she has conceived. As such, of the pregnancies we do detect, we usually note that about one-third will be normal, one-third will carry the balanced translocation, and one-third will likely miscarry from an unbalanced chromosomal translocation.

This condition is not related to a woman's age. It is relatively rare, making up less than 5 percent of all cases of recurrent pregnancy loss. Luckily, two-thirds of the known pregnancies will result in healthy live births. Prenatal chromosomal screening is usually suggested for all continuing pregnancies.