14 Facts Everyone Considering IVF Should Know About Pre-implantation Genetic Screening (PGS)

pgs-imgI thought I would share with you 14 important facts about Pre-implantation Genetic Screening (PGS), also sometimes referred to as Comprehensive Chromosome Screening (CCS), a new and important technology that has revolutionized the world of IVF. Every patient considering IVF should have PGS on their radar screen, so please take my (virtual) hand and let me walk you through it.

1. PGS was developed to address what has been one of the greatest challenges in ART, the inability of     embryologists to identify chromosomally normal (euploid) embryos for transfer into the woman’s uterus in an IVF cycle. PGS involves analyzing the cells of the embryo to ensure that it has 46 chromosomes rather than a different number. This identification is vitally important to the success of IVF as chromosomal errors in embryos lead to their failure to implant, and cause most miscarriages and all pregnancies involving chromosomal abnormalities. Only chromosomally normal embryos become healthy babies. At least fifty percent of the embryos of women older than 35 may have an abnormal number of chromosomes (are aneuploid).

2. In an IVF cycle without any form of preimplantation genetic screening, embryologists make their best assessment of which embryos are chromosomally normal. They try to inform their assessment by using a microscope to visually assess embryo morphology (how they look). But “eyeballing” embryos under a microscope is an imperfect method of assessing which embryos are the best bet for transfer. Even the most experienced embryologists are unable to identify embryos that are chromosomally normal with their eyes because chromosomally abnormal embryos often appear to be perfectly normal.

3. In an effort to develop a more precise method of screening for aneuploidy, scientists originally developed a technique called FISH (fluorescent in situ hybridization). FISH was the first technique to label chromosomes in an embryonic cell, but the problem with FISH is that only some chromosome pairs are examined. The remainder are left unexamined and this limitation is a significant drawback.

4. PGS was developed to address the limitations of FISH. It offers more “comprehensive” chromosome screening because all 23 pairs of chromosomes in each day 5 or 6 (blastocyst stage) embryo produced in an IVF cycle are examined following the biopsy of a few cells removed from the outer layer (trophectoderm) of each blastocyst.

5. Everyone considering IVF should be aware of PGS, and those in the following circumstances should be particularly aware: women who have attempted IVF without becoming pregnant more than once, women who have experienced recurrent miscarriages or have had a pregnancy involving a chromosomal abnormality, and women with diminished ovarian reserve (since a high proportion of their eggs are often chromosomally abnormal).

6. All embryos do not develop to the blastocyst stage, and generally only some of those that do will be identified as chromosomally normal.

7. Regardless of the number of blastocyst stage embryos produced, there is always a risk that none will be identified as chromosomally normal once subjected to PGS. Should this occur, a transfer will not take place.

8. Women who produce few eggs in a single IVF cycle may engage in a process referred to as “embryo banking”. In an effort to accumulate a more plentiful number of embryos for PGS analysis, their doctors stimulate their ovaries and retrieve their eggs over more than one menstrual cycle.

9. Most clinics freeze the embryos with a method called vitrification while PGS analysis is being performed. If a normal embryo is identified it is then thawed and transferred on a subsequent cycle. Some now believe that embryos are more likely to implant when the woman’s uterus has recovered from the effects of the drugs used to stimulate her ovaries. In certain circumstances, some clinics conduct the PGS within 24 hours so that if a normal embryo is identified it can be transferred fresh.

10. If chromosomally normal embryos are identified through PGS, a reputable clinic will transfer no more than two to avoid the complications that often arise in high order multiple pregnancies (triplets, quadruplets, etc.). The remainder will remain vitrified for potential transfer at a later date.

11. Even when chromosomally normal embryos are identified through PGS and transferred into the woman’s uterus, the embryo is not absolutely certain to implant. On occasion, a chromosomally normal embryo will fail to implant or miscarry due to factors unrelated to its genetic make-up.

12. In a high quality laboratory the risks of PGS are low. They include accidental damage to or destruction of the embryo during biopsy, inaccurate test results including the misdiagnosis of a chromosomally normal embryo as chromosomally abnormal and the misdiagnosis of a chromosomally abnormal embryo as normal, as well as the possibility that an embryo may not survive vitrification.

13. Because of the possibility of an inaccurate test result following PGS, all women undergoing PGS who become pregnant should speak with a genetic counsellor about whether to have prenatal testing, and the different types of prenatal tests that are available.

14. PGS is not offered at every fertility clinic and even among those clinics that do offer it, some have vastly more experience with PGS than others. Patients should be careful when choosing a fertility clinic for PGS as they will be better served by those with substantial PGS experience and high success rates in cycles where chromosomally normal embryos have been identified and transferred following PGS analysis.

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