The numerical chromosomal configuration of a cell is referred to as its karyotype or ploidy. A cell with an irregular chromosome number is referred to as aneuploid while one with a normal karyotype, as euploid. It is predominantly (but not exclusively) the chromosomal configuration of the embryo that determines its subsequent ability, upon reaching a receptive uterine environment, to propagate a normal pregnancy, also referred to as its “competence.” A “euploid” (competent) embryo transferred to a receptive uterine environment (free of anatomical, or immunologic impediments to implantation is highly likely to propagate a “viable pregnancy “
Embryo transfer (ET) is undoubtedly one of the most important variables that determines IVF outcome. The procedure itself requires gentle placement of one or more embryo(s) near the roof of the uterine cavity under direct ultrasound guidance. Central to successful IVF outcome is the selection of high quality embryos for transfer to a receptive uterine environment, ones that are the most capable of propagating a normal pregnancy (i.e. “competent embryos”). The following methods for differentiating between “competent” and “incompetent” embryos have been in use:
- Microscopic Embryo Grading using Graduated Embryo Scoring (GES):
- Embryo culture to the Blastocyst stage by day 5-6 post-fertilization:
- The Embryo Marker Expression Test (EMET)
- Preimplantation Genetic Screening (PGS) using comparative genomic hybridization (CGH) or Next Generation Gene Sequencing (NGS)
- Graduated Embryo Scoring (GES): With Graduated Embryo Scoring (GES) each embryo is separately examined through a series of microscopic assessments over a period of 72 hours following fertilization. The maximum allotted GES score is 100. Research has shown that in women <39Y, the transfer of cleaved embryos with a GES score of > 70 and are transferred to the uterus on day 3 post-fertilization, each have a 25%-30% likelihood of implanting successfully as compared to 15-20% when the GES score is <70. Comparative implantation rates are lower for older women decline to under 10% per embryo by the time the egg provider reaches 43 years of age.
- Blastocyst Transfer: The presumption has always been that it is better to transfer healthy embryos into the uterus earlier rather than later. Recent research has clearly shown this to be an erroneous belief. In fact, with few exceptions, embryos that that fail to progress to the blastocyst stage in culture are almost always chromosomally abnormal and would not propagate a healthy pregnancy anyway, even if they were to be transferred earlier. Since only about 30% of embryos progress to blastocysts by day 5-6 post-fertilization, there is a clear advantage in allowing embryos to grow to blastocysts in order to cull out many of the abnormal ones. Simply stated, the major benefit from extending embryo culture to day 5-6 is a natural selection of the best-quality embryos. This means high pregnancy rates can be achieved through the transfer of fewer embryos than usual, thus reducing the risk of high-order multiple pregnancies. It is
- The Embryo Marker Expression Test (EMET): By measuring the concentration of a genetic marker known as sHLA-G (soluble human leukocyte antigen-G), which is released into the media in which early embryos are growing after fertilization, it is possible to identify those embryos more likely to produce a pregnancy. This so called. Embryo Marker Expression Test (EMET) is performed 46 hours after the egg retrieval to identify EMET-positive, or “more likely to be competent” embryos. Levent Keskintepe PhD and I determined, based upon the performance of EMET in more than 1,000 women undergoing IVF, that the transfer of even a single EMET, positive (EMET+)-testing embryo into anatomically and immunologically receptive uteri of women under 39y resulted in better than a 40% likelihood of a viable pregnancy. In women 39y-43y the comparable viable pregnancy rate when HLA-G + embryos were transferred was about 25-30%..
- Preimplantation Genetic (PGS) Using Next Generation Gene Sequencing.
Prior to PGS, the lack of a reliable method by which to accurately assess embryo “competence” often prompted even the most well intended IVF practitioner to transfer several embryos at a time in an attempt to optimize the likelihood of a pregnancy resulting. The widespread adoption of such practice, resulted in a virtual explosion in the incidence of high order multiple births (triplets or greater). The high costs associated with addressing short term and long term, obstetric, neonatal, and social complications resulting from such high order multiple pregnancies, is/was one of the main reasons why many health insurance providers in the U.S.A. were reluctant to voluntarily cover IVF services.
The introduction of PGS into the clinical IVF arena by Levent Keskintepe and myself about 10 years ago, for the first time provided access to a method by which to relatively reliably differentiate between “competent” and “incompetent” embryos. This advance has led to:
- A lowering of the incentive to transfer multiple embryos with a commensurate reduction in the incidence of multiple pregnancies. ,
- Markedly improved IVF success rates: In the fact the transfer of even a single PGS-normal blastocyst to a receptive uterus yields about a 45-60% pregnancy rate.
- Lowering of the miscarriage rate down to <10% and
- Minimizing the chance of a chromosomal birth defects such as Down syndrome.
Many IVF programs that offer PGS embryo selection, require that all participating patients consent to all aneuploid embryos () be disposed of. However, there is now growing evidence to suggest that following embryo transfer, some aneuploid embryos will in the process of ongoing development, convert to the euploid state (i.e. “autocorrection”) and then go on to develop into chromosomally normal offspring. So clearly, by summarily discarding all aneuploid embryos as a matter of routine we are sometimes destroying some embryos that might otherwise have “autocorrected” and gone on to develop into normal offspring.
The basis for such embryo “autocorrection” lies in the fact that some embryos found through PGS-karyotyping to harbor one or more aneuploid cells (blastomeres) will often also harbor chromosomally normal (euploid) cells (blastomeres). The coexistence of both aneuploid and euploid cells coexisting in the same embryo is referred to as “mosaicism.” As stated, some mosaic embryos will In the process of subsequent cell replication convert to the normal euploid state (i.e. autocorrect).
It is against this background, that an ever increasing number of IVF practitioners, rather than summarily discard PGS-identified aneuploid embryos are now choosing to cryobanking (freeze-store) certain of them, to leave open the possibility of ultimately transferring them to the uterus.