Typically, embryo transfer is performed on the third day after egg collection, when embryos are at the 6-8 cell-stage. The decision to transfer embryos at the blastocyst stage requires special laboratory conditions, which all laboratories are not able to meet. Blastocyst-stage transfer is indicated in the following cases:

·           when previous attempts of day-2 or day-3 embryo transfer have inexplicably failed,

·        when there is an excess of day-1 to day-3 embryos with comparable embryo morphological score. In such cases, embryo transfer may be delayed and embryos kept in culture up to the blastocyst stage, thus enabling embryologists to choose the most competent ones for transfer.

·          when performing preimplantation diagnosis. In these cases, the embryo is biopsied on day 3 or day 5 of culture and embryo transfer is necessarily performed on day 5 or on day 6, respectively (to make time for communication of PGD results by the geneticists).