About a decade ago, I, along with my associate, Levent Keskintepe PhD were the first to introduce full chromosome Preimplantation Genetic Sampling/Screening (PGS) into the IVF clinical realm to try and identify euploid embryos whose cells contained the required 46 chromosomes (23 pairs) necessary to render them potentially “competent” to propagate viable pregnancies. Aneuploid embryos (those that have more or less than a total of 46 chromosomes) are by and large considered to be “incompetent”, far less likely to propagate a viable pregnancy and thus largely unworthy of being transferred to the uterus.
Initially the primary method used for PGS was, comparative genomic hybridization (CGH). The methodology was not without certain problems. A few years ago, new and improved technology known as next generation gene sequencing (NGS) emerged. This has since all but replaced other methodologies. Gene sequencing determines the precise order of nucleotides within a DNA molecule. It includes any method or technology that is used to determine the order of the four bases—adenine, guanine, cytosine, and thymine—in a strand of DNA.
The widely held belief is that the ideal time to biopsy embryos for PGS is when they reach the most advanced stage of preimplantation development (the blastocyst stage) by 5-6 days post-fertilization. At this point several cells are microsurgically removed from the embryo’s outer cellular layer (trophectoderm-TE), processed and subjected to PGS analysis. The blastocysts are ultra-rapidly frozen (vitrified) and held for future dispensation in a subsequent frozen embryo transfer (FET) cycle, once test results are known.
Access to several cells through TE biopsy provides more DNA for reliable analysis that can be attained through the testing of a single cell removed from a day-2-3 cleaved embryo. It is this plus the belief that the hypercellular blastocyst is far less likely to be damaged through such microsurgical intervention than would be the case with a 4-10 cell, day-3 cleaved embryo that has led to the preferred timing for biopsy to be on day 5-6 blastocysts..
When PGS testing was first introduced, initial results were most-encouraging. Embryo implantation rates of >50% and birth rates of 50-60% when up to two euploid blastocysts were transferred, were being reported. In addition, the reported incidence of miscarriages and chromosomal birth defects was likewise greatly reduced. In fact, we were so encouraged that most of us predicted that a time would come where full embryo karyotyping through PGS would become a routine part of IVF. But alas, we were soon to be disappointed when following the widespread introduction of PGS testing success rates started dropping. This was especially the case when PGS was performed on embryos derived from the eggs of older women and women with severely diminished ovarian reserve (DOR). With further investigation it began to dawn upon us that:
- Chromosomal numerical integrity, while being the most important determinant of embryo “competency” was likely not the only factor that impacted embryo “competency”. Indeed advancing age was revealed to increase the incidence of embryo aneuploidy, independent of embryo karyotype and this is probably linked to non-chromosomal, genetic and metabolomic factors that might also be age-related.
- Independent of embryo competency, there are many variables, that can and also do determine IVF outcome and these are often outside the control of the embryology/genetic laboratory. They include selection and implementation of individualized protocols for controlled ovarian stimulation (COS), endometrial factors that determine embryo implantation (e.g. anatomical an immunologic implantation dysfunction), technical skill of the physician performing embryo transfer etc.
- Not all PGS-aneuploid embryos are “incompetent”. Some are mosaic (see elsewhere) and these are often capable of “autocorrecting” upon being transferred to the uterus, and propagating healthy babies.
Example A: Under optimal conditions embryo “competency” is determined by age and the protocol used for COS. In women <36Y of age roughly 1:2 blastocysts will likely be euploid “competent” and were such an embryo be gently and expertly transferred to a “receptive” uterine environment, the chance of a viable pregnancy should about 55-60%. This means that when ET is performed in such ideal IVF candidates, the chance of it resulting in a live birth should be about 27%-30% per embryo.
Example B: Conversely, when it comes to a woman in her mid-forties, the chance of an embryo being “competent” is probably < 1:8-10. And, the age-adjusted chance of such a Euploid embryo propagating a live birth is (for reasons cited above) theoretically well below 60% (perhaps around 40%-45%). This extrapolates to a baby rate of no more than 4%-5% per blastocyst transferred.
Using the above examples:
In Example A: Given that about 50% of the eggs (and thus resulting embryos) of young women are euploid and competent, the transfer up to 2 non-PGS tested blastocysts would likely yield the same results as would the transfer of a single PGS-tested euploid blastocyst. It follows that a patient/couple who are capable and willing to engage a twin pregnancy (which would occur in roughly 25% of such cases), might get as good a result by simply transferring two (2) untested blastocysts and in the process avoid the additional cost of PGS.
In Example B: Conversely, the chance of a viable pregnancy in a woman in her mid-40’s would likely be 8-10 times greater when a “competent”, PGS-euploid blastocyst is selectively transferred as compared to when a non-PGS tested blastocyst is transferred to the uterus (4% versus 40%). Albeit that PGS-testing of blastocysts derived from fertilization of an older woman’s eggs is less reliable than for younger counterparts, there would be a distinct benefit/advantage in pre-selecting euploid, “competent” blastocysts for transfer in such cases. Since older women often also have DOR and thus produce fewer eggs/embryos, such women should benefit inordinately from the selective “stockpiling” (banking) of PGS-biopsied blastocysts (vitrification) over several cycles of IVF for collective PGS testing and the subsequent selective transfer of only the most “competent” ones to the uterus with FET.
In conclusion, it is my considered opinion that PGS-embryo selection only be considered in the following circumstances:
- Women over the age of 39Y and those who, regardless of age have significant DOR, are running out of eggs and time, and need to “make hay while the sun shines”!
- Unexplained IVF failure.
- Certain cases of recurrent pregnancy loss (RPL).
- Family gender balancing cases
- Women who have alloimmune implantation dysfunction (IID) with activation of uterine natural killer cells (NKa)
- Where karyotyping reveals one or other partner to have a balanced chromosomal translocation
- Known or anticipated specific genetic abnormalities
When selectively used PGS is an excellent tool to improve implantation potential and IVF outcome (see above). While PGS provides a new and unique method for selecting the ideal embryos to be transferred, it is not a panacea when it comes to identifying “competent embryos”. There are factors other than numerical chromosomal integrity (karyotype) that can and do influence embryo “competency”, profoundly. PGS embryo selection is in my opinion currently over-used. This is especially the case when it comes to younger women with normal ovarian reserve. Unless the dust is allowed to settle such that this remarkable technology is properly applied, it is my belief that it stands the risk of progressively falling into disrepute.