Obviously, 3 through 6 are gametes that have the incorrect number of chromosomes (either too little or too much genetic material). When these gametes get together with the, presumably, ‘normal’ gametes from her partner (which contain one free copy of chromosome 13 and 14), the following possibilities result (in the same order as above):
Therefore, to answer one of our reader’s questions, mathematically, she has only a 2 in 6 (33.3%) chance of having a baby that has the right TOTAL amount of genetic material; one of these will be entirely chromosomally normal and the other will be a translocation ‘carrier’ just like herself. Two-thirds of her babies are at risk for being chromosomally ABNORMAL.
But, as I mentioned at the outset, things are not quite that simple. Indeed, her actual risk for having a baby with a chromosomal abnormality is much lower than this. The monosomy 13 and 14 embryos will not be successful at all and the trisomy 14 embryos also have very little chance of surviving much of the first trimester. Most trisomy 13 embryos will also be lost early in first trimester and the few that survive will have only a small chance of surviving the pregnancy and even a smaller chance of living more than a few hours or days after birth. These babies all have severe congenital malformations and if they manage to survive birth and the neonatal period, profound metabolic disturbances, and mental retardation. Indeed, the ‘selective forces’ are so strong against these chromosomally abnormal conceptuses that at least two-thirds of her pregnancies in which a pregnancy is actually confirmed will be chromosomally normal and the chances of actually DELIVERING a chromosomally abnormal baby are probably only about 1%! The overall risk of miscarriage is about 25%.
With regard to the question of IVF (in vitro fertilization) and PGD (prenatal genetic diagnosis), I have a significant amount of ambivalence. If you have the money to burn, these are certainly options, but they are very expensive procedures. And, if you have no difficulty conceiving and are willing to trust nature to do the right thing, as pointed out above, the risk for actually having a baby with an unbalanced karyotype is so small, that it is often simply waiting until the dice roll correctly to have a normal baby. I know that can be difficult psychologically and at times physically. However, because there is nothing that can be done to correct a translocation, if one can accept the fact there is an increased risk for miscarriages, and that when these occur, it is probably the result of an aneuploid fetus, dealing with the pain of pregnancy loss may be a little bit easier.
There is a lot we have not discussed about Robertsonian translocations in this post, but I would like to make a few recommendations in closing (and we can always use these other issues as an excuse to write another post). First, I would suggest that any couple with a known Robertsonian translocation, consider having combined first trimester screening for aneuploidy performed and seriously consider a chorionic villus sampling if this is 'abnormal'. Secondly, even if this is reassuring, consider having a fetal karyotype done by amniocentesis. Thirdly, once an individual with a Roberstonian translocation has been identified, I think it is important to let other family members (male or female) of reproductive age know so that they can be screened as well. It may save a lot of anguish down the line.
Anyway, Gina, thanks again for your questions. I am wagering right now that your pregnancy quest will be rewarded in the end. Best of luck to you and your husband and thanks for reading!