Introduction The goal of assisted reproductive technology (ART) is the delivery of a healthy singleton, with the treatment strategy in IVF having the best chance being the transfer of the single most viable embryo from a patient's embryo cohort. Embryo selection, therefore, is paramount to the success of this goal. The microscopic assessment and scoring of embryo morphology was developed as an embryo selection technique in the early years of IVF (Veeck, 1991) and has ever since remained the most widely practised embryo selection technique. However, the technique's limited ability to maximise embryo implantation rates has for long been lamented and, therefore, more enhanced embryo selection techniques have for long been sought. Time-lapse monitoring and scoring (Paulson et al., 2018) and euploid embryo transfer using PGT-A (preimplantation genetic testing for aneuploidy; Gleicher et al., 2017) are recent innovations that have become contenders to replace conventional microscopic assessment and scoring of embryo morphology. An advantage of PGT-A is its independence of embryo development and morphological feature assessments, as euploid blastocysts implant at the same rate irrespective of blastocyst morphology score (Capalbo et al., 2014). Moreover, besides PGT-A's technical and biological limitations, the costs of implementation PGT-A as an embryo selection technique may be regarded prohibitive by many.

In the evolution of PGT in human IVF, PGT has progressed from PGT-v1 to PGT-v2 based on the adverse outcomes of biopsying 1-2 blastomeres from cleavage-stage embryos (PGT-v1) and the improvement in blastocyst development conditions. The evidence suggested that PGT-v.1 posed a significant risk to embryo viability, with no benefit resulting from the transfer of euploid embryos (Gleicher et al., 2017). The majority of studies have reported that PGT-v2, which requires the biopsying of 5-10 trophectoderm (TE) cells, poses no significant risk to the developmental potential (implantation viability) of biopsied blastocysts. However, recently there have been reports, suggesting that TE biopsy may not be completely free of risk (Gleicher et al., 2017, Ozgur et al., 2019). Currently, PGT-A faces four challenges preventing its routine use as a blastocyst selection technique; (1) the need for high technical expertise, (2) the invasiveness of TE biopsy, (3) the cost of the full technique, and (4) that TE biopsy is subject to sampling bias - a single TE biopsy of 5-10 cells may not accurately represent the ploidy of the whole blastocyst.

A spin-off of conventional PGT is the analysis of DNA in blastocoel fluid (BF). This phenomenon was first reported by Palini et al. (2013). The origin of the DNA in BF has been suggested to be the result of embryo euploidisation, i.e., the elimination of aneuploidy DNA or aneuploidic cells through regulatory processes such as apoptosis and or necrosis (Leaver and Wells, 2019). The practice of blastocyst collapse before vitrification (Mukaida et al., 2006, Iwayama et al., 2011) provides the opportunity to perform minimally invasive PGT-A (miPGT-A), i.e., the biopsying BF. In a study to investigate ploidy concordance between PGT-A with TE biopsy and PGT-A with BF biopsy, the authors found that the clinical pregnancy rate was 77% for blastocysts with no DNA amplification were transferred and 37% for blastocysts with DNA amplification (Magli et al., 2018). The mere accumulation of DNA in the BF, therefore, could be used as a supplementary measure to prioritise blastocysts for transfer. Implementing BF-biopsy and whole-genomic amplification (WGA) as a supplementary selection measure limits/reduces three of PGT-A's main challenges; (1) technical expertise, (2) invasiveness, and (3) costs.

In the present randomized controlled trial (RCT), the investigators will investigate DNA amplification in blastocyst fluid biopsies (BF-biopsy) as a supplementary selection technique to select blastocysts for transfer in conjunction with blastocyst morphology scores. The clinical implantation outcomes of FETs in which score-selected single blastocyst with no DNA amplification and score-selected single blastocysts were transferred will be compared.