These are frozen embryos from a former cycle wich can be used in a new…
The prolonged cultivation of embryos during the five days following fertilisation allows us to better select the embryos that will be transferred into the uterus. This is the case because certain embryos block their division on the third day of development. With this technique we ensure that the transferred embryos have overcome this block, and therefore we increase the rate of embryo implantation, and consequently the gestation rate.
One of the requirements is that the number of resulting embryos after fertilisation be high, given that they will not all reach the blastocyst stage and they will not all be good quality, which is why with a small number of embryos the embryo transfer would be carried out on day 2 or 3.
Pre-embryo 16-20 hours after fertilisation (Day 1)
4-cell embryo 48 hours after fertilisation (Day 2)
8-cell embryo 72 hours after fertilisation (Day 3)
Compacting embryo 80 hours after fertilisation (Day 3)
Embryo in blastocyst stage 120 hours after fertilisation (Day 5)