on the uselessness of the first polar body for this purpose.104 Various approaches were explored in the attempt to visualize the chromosomes of the first and second polar bodies, as well as of the biopsied blastomeres, including nuclear transfer, electrofusion, and chemical methods.102, 105, 106 However, the major progress in chromosome analysis of oocytes and embryos was achieved with introduction of the fluorescence in situ hybridization (FISH) technique,107–113 microarray technology, and NGS. Our meiosis data based on the analysis of 22,986 oocytes detected 9,812 aneuploid oocytes (46.8 percent), originating comparably from the first and second meiotic divisions. Overall, meiotic division errors were observed in 33.1 percent of oocytes in meiosis I, 38.1 percent in meiosis II, and 28.8 percent in both.
Although the aneuploidy rate in embryos is comparable to that in oocytes, the types of anomalies in the oocytes and embryos were significantly different,114–116 also showing inconsistency between the expected and observed frequency of some types of aneuploidies. In our current practice of PGT‐A, the analysis of 2,922 blastocysts, 56.0 percent of embryos were aneuploid, comprising 13.0 percent monosomy, 13.0 percent trisomy, 8.0 percent numerical mosaic, 14.0 percent segmental mosaic, and 8.0 percent complex errors.48 It is of interest that no age dependence was revealed for monosomies in embryos, suggesting that the rate of monosomies detected in embryo by PGT‐A may be of artefactual nature.
A possible explanation for this discordance is that the majority of monosomies detected in embryos are derived from mitotic errors, assuming technical causes are excluded. In fact, a significant proportion of the cleavage‐stage monosomies appeared to be euploid after their reanalysis.117, 118 No monosomies, except monosomy 21, are detected after implantation, so either they are eliminated before implantation or have no biological significance, reflecting the poor viability of the monosomic embryos and their degenerative changes. In one relevant study the progression and survival of different types of chromosome abnormalities were followed up in 2,204 fertilized oocytes.119 A variety of chromosome abnormalities was detected, including many types of errors not recorded later in development. However, these appeared to be tolerated until activation of the embryonic genome, after which there were declines in frequency. Nevertheless, many aneuploid embryos still successfully reach the blastocyst stage, even if some chromosome errors present during preimplantation development are not seen in later pregnancy.
Aneuploidies
As seen from the previous discussion, without the detection and avoidance of the transfer of chromosomally abnormal embryos, there is at least 50 percent chance of loss during implantation or postimplantation development. In addition to the clear benefit of avoiding the transfer of aneuploid embryos, which contributes to improvement of pregnancy outcome of poor‐prognosis IVF patients, this should improve the overall standard of medical practice, upgrading the current selection of embryos by morphologic criteria into aneuploidy testing. This explains a widespread application of PGT‐A aimed at the preselection of embryos with the highest developmental potential demonstrating a clinical benefit, in terms of the improved IVF outcome through improved implantation and pregnancy rates, reduction of spontaneous abortions and improved take‐home baby rates in IVF patients of advanced reproductive age, repeated IVF failures, and recurrent spontaneous abortions.24, 25, 114–116, 116, 120
The failure to detect a positive effect of aneuploidy testing on reproductive outcome in a few studies may be due to possible methodologic deficiencies.122–124 Despite these methodological shortcomings, which have been heavily criticized in the literature,125–127 the American Society for Reproductive Medicine Practice Committee did not favor transferring embryos without aneuploidy testing.128 This may mean the alternative of incidental transfer of chromosomally abnormal embryos, as every second oocyte or embryo obtained from poor‐prognosis IVF patients is chromosomally abnormal and destined to be lost before or after implantation. In fact, only one in ten of chromosomally abnormal embryos may survive to recognized clinical pregnancy, 5 percent may survive to the second trimester, and only 0.5 percent reach birth. Thus, the majority of chromosomal abnormalities are eliminated before or during implantation, reflecting a poor implantation rate in poor‐prognosis IVF patients, and explaining a high fetal loss rate in those patients without PGT‐A. This has actually been demonstrated by testing products of conception from poor‐prognosis non‐PGT IVF patients, confirming the high prevalence of chromosomal aneuploidy in the absence of PGT‐A. Of 273 cases tested, 64.8 percent had chromosomal abnormalities, up to 79 percent of which could have been detected and not transferred using PGT.129
Given these data, the current IVF practice of selecting embryos for transfer based on morphologic criteria may hardly be an acceptable procedure for poor‐prognosis IVF patients. In addition to giving an extremely high risk of establishing an affected pregnancy from the onset, this will significantly compromise the very poor chances of these patients to become pregnant,130, 131 especially with the current tendency of limiting the number of transferred embryos to only a few or even one. Although culturing embryos to day 5 (blastocyst) before transfer may allow, to some extent, the preselection of developmentally more suitable embryos compared with day 3, some aneuploid embryos will still be capable of developing to the blastocyst stage.132, 133 These abnormal embryos will not be eliminated in the current shift to blastocyst transfer, and may implant and lead to fetal loss, compromising the outcome of pregnancies resulting from the implanted normal embryos in multiple pregnancies. In fact, multiple pregnancies represent a severe complication of IVF, which is currently avoided by preselection and transfer of a single aneuploidy‐free blastocyst with the greatest developmental potential to result in a healthy pregnancy.
However, contrary views also exist about safety, outcome, and efficacy.122–124, 134–137 Randomized controlled studies performed with introduction of next‐generation technologies were able to quantify the clinical impact of preselection of aneuploidy‐free zygotes, demonstrating the obvious benefit approximating a 15–20 percent increase in pregnancy rates, compared to embryos transferred solely based on morphological criteria, although this was not universal in all age groups.
The first randomized controlled trial (RCT) using 24‐chromosome analysis was performed in a series of 112 women randomized into two groups:138 transfer of a PGT‐A embryo versus transfer of a morphologically normal embryo not biopsied or tested. Of 425 blastocysts tested, 45 percent (191/425) were with aneuploidy, resulting in a 71 percent pregnancy rate compared with 46 percent in the nonbiopsied control group of 389 blastocysts with normal morphology. In the other RCT, involving 72 cases, the transfer of euploid blastocysts resulted in 66 percent implantation and 85 percent delivery rates, compared to 48 percent and 68 percent, respectively, in the control group of 83 morphologically normal embryos but not tested for PGT‐A.139 Another RCT did not find differences in pregnancy rates between single euploid embryo transfer and the transfer of two morphologically normal but untested embryos, but a 48 percent twin rate was observed in the latter compared to 0 percent in the single embryo transfer tested group.140 Significant differences between a PGT‐A group and control groups were also demonstrated in an RCT performed using a cleavage‐stage embryo biopsy.141 Thus, results of RCTs involving 24‐chromosome platforms suggest that it is reasonable to inform assisted reproductive technology (ART) patients of advanced maternal age about the utility of PGT‐A. The precise age range at which women would benefit is still under study, although the optimal outcome seems to be for the 35–39 age group, as suggested by trials conducted by the Society for Assisted Reproductive Technology (SART)142 and the STAR Study Group.143
The switch of aneuploidy testing from FISH to the next‐generation technologies for 24‐chromosome testing,144–163 allowing improved detection of chromosomally
