Cryopreservation of human embryos is a safe procedure, which has been carried out for more than last 30 years. In development of in vitro techniques and together with single embryo transfer becoming greater demand for an efficient and reliable cryopreservation method for surplus embryos. It is possible to cryopreserve the human zygotes immediately after fertilization, at the pronuclear stage or embryos during early cleavage stages (2–8 cells) or at the expanded blastocyst stage (after 5–7 days in culture).
Embryos are cryopreserved in any embryonic stages. Still there does not exist a common consensus what is the most optimal developmental stage for embryo cryopreservation.
Since morphology of vitrified and thawed embryos is not enough to assess the viability, the possibility of culturing for a few more days before transfer can ensure that embryo is for transfer. In contrary to oocytes, embryos are after cortical reaction, which gives the ooplasmic membrane more stability to cope with the low temperature and osmotic changes.