LifeQuest Articles

In this issue of LifeQuest's newsletter, we are very pleased to provide a detailed overview of PGD (Preimplant-ation Genetic Diagnosis) and PGS (Preimplantation Genetic Testing), genetic tests that are among the latest Assisted Reproductive Technologies available to couples undergoing IVF. LifeQuest manages more PGD/PGS cycles than any other fertility clinic in Canada. And we do so at our dedicated, on-site human embryology laboratory. We hope you find the following article interesting and informative. As always, if you have additional questions, or to find out if this treatment is right for you, please talk to your LifeQuest physician.  

Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Screening (PGS), are genetic tests that help identify embryos suitable for transfer back to the uterus in an IVF cycle. LifeQuest manages more PGD/PGS cycles than any other fertility clinic in Canada. And we do so at our dedicated, on-site human embryology laboratory. The term “PGD” is often used as a catch-all phrase for all cases where a cell is removed from an embryo for genetic testing prior to placing or transferring the embryo back to the uterus. The distinction between PGD and PGS can be confusing and is not consistently used. Those patients with a known genetic abnormality such as a single gene defect, undergo PGD while those for whom this technology is a tool to help screen embryos for abnormal number of chromosomes or aneuploidy (as in the case of women over the age of 38 or with a history of recurrent pregnancy loss) are said to be having screening done, hence the use of the term PGS.

For whom is this testing appropriate?

• Women of advanced maternal age (>38years) may benefit from aneuploidy testing (testing for abnormal chromosome number) (PGS) but the literature is still pending on reliability of this testing procedure. (PGS is the concept whereas FiSH and CGH are the techniques used in screening). Newer technology called comparative genomic hybridization (CGH) is much more reliable than the older FiSH technology used about two years ago. In addition, unlike the older FiSH technology where only 12 pairs of chromosomes could be tested, CGH can test for all 23 pairs or 46 chromosomes. In theory, as a woman gets older the chance for aneuploidy in the embryo increases markedly. This is due to an aging issue in the egg. PGS can detect this level of aneuploidy in the embryo and allow the transfer of only those embryos with the proper number of chromosomes.  An embryo with an improper number of chromosomes cannot always be detected visually by the embryologist. CGH, although not perfect, can give us a more accurate chromosomal picture of the embryo.

• Couples with a history of unexplained recurrent pregnancy loss (miscarriage). In couples where molecular, genetic, chemical, and mechanical testing has not found any abnormality, a possible next step is to examine the number of chromosomes in the embryo. In recurrent pregnancy loss (miscarriage), the embryo may have an abnormal number of chromosomes (often the smaller chromosomes such as 13, 18, 20 etc,) so that “nature” does not see an abnormality until 5 –7 weeks of pregnancy at which time the pregnancy stops progressing. For these patients, PGS can be useful in detecting the normal embryos and implanting only those that have a normal number of chromosomes.

• Couples with a history of a previous pregnancy that was found to be chromosomally abnormal. In these cases, PGS (CGH) would be the testing procedure used as described above.

• Couples where at least one partner has a known genetic defect (such as cystic fibrosis, Tay Sachs, myotonic dystrophy, etc) that has previously been identified. Technology now allows the genetics lab to screen the removed or biopsied cell for up to 1500 mutations. However, this can still only be done one gene at a time and, in addition, the genetics lab requires at least 6 weeks to make the tool or probe to actually detect the abnormal gene in the cell. Therefore, if you or your spouse are carriers of a genetic disease, your embryos can be screened for the particular gene and only those embryos without the gene will be transferred back to the uterus.

• Couples where at least one partner has a specific chromosomal rearrangement known as a translocation. Translocations can cause a pregnancy to abort or cause an abnormality in the newborn. Therefore, if a translocation is detected during karyotype testing, PGD may be indicated in the IVF cycle to avoid or minimize the effect of the chromosomal rearrangement.

• Couples who desire an HLA-matched sibling for a living child requiring a bone marrow transplant. Usually, in these cases, the gene afflicting the child will be screened and the embryo not expressing this gene will be considered for transfer in the IVF cycle. It is also possible to tissue type an embryo and implant an embryo that is a tissue match for the ailing sibling.