Time for embryo transfer
The best time for embryo transfer is determined by their number, respective quality, sensitivity to in vitro culture and desire or a need for genetic testing.
The stage of development for embryo transfer is important and the comprehensive discussion is difficult because there are too many possibilities. Therefore, it is only possible to provide some general guiding principles.
First, it is important to understand that 1. extended embryo culture (beyond fertilization) is only required for better embryo selection 2. it can’t improve embryo quality, but can fail some otherwise viable embryos that would continue development if transferred without extended culture.
Therefore, in theory, any culture beyond zygote must have an indication (justification).
A number of embryos and the day of transfer:
If a patient only has a single fertilized egg, there is nothing to select, and therefore that are no benefits of extended culture or PGT, which requires extended culture. One could argue that if embryos are left in culture and become arrested, this is because it was not viable and the patient saves money by avoiding a transfer that would fail anyway. However, how can we be sure that the same embryo would fail to implant if transferred earlier? The evidence that earlier transfer may rescue embryos is not merely anecdotal and concur with my own experience.
Generally, the more embryos a patient has, the more likely that extended culture is indicated.
Embryo quality and day of transfer
There is no better place for the embryo than the uterus, therefore if the zygote or day 2 embryos quality does not look promising, the early transfer should be given consideration. Generally, the worse embryo quality, the less benefit of extended culture.
Embryo sensitivity to in vitro culture
Above-average embryo sensitivity to in vitro culture can only be evaluated retrospectively when apparently good quality embryos failed to develop to the blastocyst stage. Earlier transfer in such cases for the next round of IVF should be considered.
Genetic testing and extended culture
Genetic testing of embryos is impossible without the extended culture. If PGT is imperative, then there is no other choice than culturing embryos to the blastocyst stage in vitro.