Fresh embryo transfer is performed 5 days after follicular puncture, when the embryos have reached the blastocyst stage.
In the transfer of frozen embryos, these are also frozen at the blastocyst stage and transferred in a different cycle, after being previously thawed.
In common practice we prefer to transfer the fresh embryos as the success rates are slightly higher but sometimes it is necessary to freeze them to transfer them in a later cycle, without significantly affecting the patients' chances of pregnancy.