Complex mosaic blastocysts after preimplantation genetic testing: prevalence and outcomes after re-biopsy and re-vitrification

What is the incidence of complex mosaic in preimplantation genetic testing (PGT) blastocysts and can it be managed in clinical practice? A retrospective study of PGT cycles conducted between January 2018 and October 2019 at a single centre. Biopsies of blastocysts were collected and analysed by next-generation sequencing (NGS). Complex mosaic blastocysts were defined as those with three or more mosaic chromosomes. The cryopreserved complex mosaic blastocysts underwent a second round of biopsy, NGS analysis and vitrification. The euploid blastocysts identified by the re-biopsy were warmed again for embryo transfer. The main outcomes included the prevalence of the complex mosaic and the ongoing pregnancy rate. The prevalence of the complex mosaic was 2.4% (437/17,979). The prevalence of the complex mosaic was not associated with maternal age and morphological quality. A total of 89 complex mosaic blastocysts underwent re-biopsy and 96.6% (86/89) survived the first warming. For the re-biopsy samples, 61.6% (53/86) were euploid. The poor-quality blastocysts had higher rates of aneuploidy compared with good-quality blastocysts. The survival rate for blastocysts undergoing the second warming was 100% (18/18) and resulted in an ongoing pregnancy rate of 38.9% (7/18) as well as the birth of six healthy infants. Re-biopsy may rescue blastocysts with development potential for transfer and improve the cumulative pregnancy rate per stimulation cycle in patients containing complex mosaic blastocysts.