A “competent” embryo is one that is chromosomally, genetically and metabolically capable of propagating a viable conceptus in-utero. Such an embryo will have the same chance of resulting in a baby whether it is transferred cleaved( on day 2,3,4) or as a blastocyst (day 5 or 6), post-fertilization. Currently, we have no ability to reliably assess the genetic and metabolic integrity of an embryo. The only tools we currently have to rely on are: a) the embryo’s microscopic appearance and its rate of cell division (cleavage ate) and, b) its numeric chromosomal integrity as determined by preimplantation genetic testing  or PGT (the introduction of which, I proudly played a significant role >15 years ago). It is important to understand that embryos that fail to reach the blastocyst stage of development in culture, by day 5-6 post-fertilization are invariably “incompetent” and are thus unworthy of being transferred.  There is absolutely no truth to the assertion that cleaved embryos benefit through being transferred to the uterus earlier than the blastocyst stage of development or that cleaved embryos which otherwise would not develop into blastocysts while in culture, would have an improved chance of progressing to the blastocyst stage had they been transferred to the uterus earlier on. This is the reason why in most cases, enlightened IVF practitioners,  encourage their patients to keep their embryos in culture for 5-6 days before even considering them for transfer or deciding to subject them to biopsy for PGT to better assess their “competency”. But PGT, while an important advance, is only part of the story as it does not totally define embryo “competency”. It is simply a selection tool by which to  identifying and select  the best embryos for transfer or for discarding “incompetent” embryos. The first step in assessing embryo “competency” is proof of normal fertilization, the second is the embryo’s cleavage rate and microscopic appearance. The third is the ability of the embryo to reach the expanded blastocyst stage of development by day 6 post-fertilization and the fourth and final test of embryo “competency”, is  PGT analysis. Therefore, PGT does not enhance pregnancy cumulative pregnancy rate. Rather by taking an embryo through all 4 steps of “competency” it allows for enhanced selection of the one (or two) that are to be transferred. The ability to better grade embryos “competency” by challenging their to reach the blastocyst stage of development in culture and then selectively performing PGT -analyses, provides an opportunity to improve the baby rate per embryo/blastocyst transferred and in the process allows for the transfer of fewer embryos at a time and thereby reduces the risks associated with multiple pregnancies. There are numerous variables other than embryo “competency” that impact the likelihood of success or failure with IVF. The absence/presence/severity of anatomical or immunologic implantation factors , laboratory expertise and skill of the practicing physician are but a few . This makes it impossible to confidently predict success rates with IVF based solely upon the number of cleaved embryos or blastocysts transferred or even the results of PGT. It is of course self-evident that the further along the embryo is in development as well whether its  chromosomal integrity has been established (through through PGT) , all should contribute to making a the decision as to  how many to transfer at a time. In my practice, because of the risks associated with the inordinately high risks to mother and babies, associated with triplets or greater (high order multiple pregnancies,) I very rarely transfer more than 2 embryos at a time, whether they are cleaved, blastocysts or PGT-normal-euploid. It is well to remember that whether you transfer 1 or >1 embryo at a time, in the final analysis, the overall number of babies yielded will likely be the same.  It might require that the woman undergo more embryo transfers to have a baby, if only 1 embryo is transferred at a time, but the incidence of multiple pregnancies with the associated risks to mother and offspring will be much lower.