When it comes time to thaw the embryos, all available identifiers of the stored specimen must match and be confirmed before thawing commences. The embryos are thawed out in a minute or two, more or less, at room temperature. However, the time-consuming element of the thaw procedure is not the timing but the careful dilution of the cryoprotectant fluid to return the embryo to its favored culture medium. This permits resumed growth and development in vitro. Once this is done, the embryo is assessed for cryodamage to determine if it is suitable for transfer.
Embryos can accommodate such levels of cellular damage and still establish healthy pregnancies.
All thawed embryo routinely undergo assisted hatching prior to transfer. The zona pellucida , which surrounds the embryo, has been shown to suffer a certain amount of hardening during cryopreservation. This can be overcome by artificially making an opening in this outer embryo shell. Varying strategies may be applied according to how many and which embryos are thawed prior to transfer. Indeed, most couples have only one or two embryos frozen, so that all are thawed, and any surviving are replaced.