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The impact of PGT is even higher in translocation patients, with considerable reduction in the spontaneous abortion rate after preimplantation testing, resulting in a corresponding increase in the take‐home baby rate.^{130, 131} Although previous experience with PGT for chromosome structural rearrangements (PGT‐SR) was based on the use of the FISH technique, which is still applicable in some specific cases, the current standard is the utilization of array‐CGH or NGS technologies, which improves accuracy of testing and also allows a combined PGT‐A. An example of PGT‐SR by NGS is shown in Figure 2.5.

Figure 2.5 Next‐generation sequence‐based testing for translocation 46,XX, t(6;18)(p21.3;p11.2) (derivative chromosome indicated by red arrows).

In our experience of 940 PGT‐SR cycles, the comparison of reproductive outcomes of 609 cycles performed by FISH and 331 performed by array‐CGH and NGS showed significant improvement of the application of next‐generation technologies, resulting in almost doubling pregnancy rate, from 38.8 percent in FISH cycles to 66.5 percent with application of next‐generation technologies, and twofold reduction of spontaneous abortion rate, from 18.1 percent to 8.9 percent.⁴⁸

A few sophisticated approaches based on next‐generation technologies have been developed for distinguishing noncarrier balanced embryos from normal ones. One such technology involved the use of an SNP microarray.^{166, 167} However, this method requires the availability of a lot of the unbalanced embryo, as well as parental DNA necessary to serve as a reference for distinguishing balanced translocation from normal blastocysts. The more universal approach is a specially designed NGS technology called mate‐pair sequencing (MPS). This involves high‐depth MPS to identify breakpoint regions and Sanger sequencing to define the exact breakpoint needed for designing specific primers required to identify normal and carrier embryos.¹⁶⁸ A similar approach, termed nanopore long‐read sequencing, also discriminates carrier from noncarrier embryos through high‐resolution breakpoint mapping followed by breakpoint PCR.¹⁶⁹ Thus, following application of breakpoint PCR, carrier embryos can be discriminated from noncarrier embryos. Both these approaches enable accurate high‐resolution breakpoint mapping directly on balanced reciprocal translocation carriers, providing the option of transferring euploid noncarrier embryos. Thus, the current technologies not only insure an acceptable pregnancy outcome for carriers of structural rearrangement, but also enable avoidance of balanced offspring and continuation of the problem in the next generation.

The presented data provide strong evidence that PGT is currently an important alternative to prenatal diagnosis, as it widens the options available for couples wishing to avoid the birth of an affected child, and provides the possibility of having children for those who would remain childless because of their objection to termination of pregnancy following prenatal diagnosis. At the same time, PGT is also becoming an integral part of assisted reproduction, by avoiding transfer of chromosomally abnormal and potentially nonviable embryos, thereby contributing to a significant increase in implantation and pregnancy rates in IVF, and to a general improvement in the standards of assisted reproduction practices.