In IVF, the routine policy is to take the embryo to a blastocyst stage, that is a day five stage. The uniqueness of the blastocyst stage is that embryos start dividing at a faster rate. Until day four they are dividing in very much an arithmetic progression of doubling while between day four and day five they go into a far more accelerated growth and division of the cells taking place with cells which are called the inner cell mass which forms the baby, and the outer cell mass that forms the placenta, thus at this stage the embryo is ready to start the process of implantation.
There is good evidence to suggest that embryos that do not reach this stage may not be of sufficient quality to implant. Often we are told that it is possible that embryos that do not reach this stage could be genetically abnormal. Whilst this has been based on hearsay evidence, there has been a study which looked at screening these embryos through a biopsy when they did not reach a blastocyst stage, which is a morula stage on day six. This was a study undertaken and published in Human Reproduction 2018, and routinely morulas, which are day four embryos which never became blastocysts, even by day six (while they should become by day five a blastocyst) were biopsied and sent for genetic analysis. These were exactly the embryos which were slowly progressing.
A huge number of morulas were biopsied, close to 1260 and 3014 blastocysts were biopsied. What was shown is that blastocysts which had an aneuploidy under the age of 35, 40 per cent of good blastocysts are abnormal, while by the age of 40, almost 78 per cent of blastocysts are abnormal, but when you started doing a biopsy of embryos which were slow, it identified that the abnormality rate ranges between 57 to 68 per cent, and this worsened as women reached 38 to 39 and 80 per cent of these embryos were abnormal.
Thus there is no doubt that with advancing age and embryos not reaching a blastocyst stage, the percentage of embryos which are abnormal increases, thus there is some logic towards not transferring these embryos in older women, while in younger women it may be reasonable to try and see whether transferring these in a fresh cycle would give a better chance of pregnancy.