There are many reasons why embryos are frozen. For example, in some IVF cycles, multiple embryos are of good quality. Rather than discard these embryos, they are frozen for future use. Embryos must also be frozen in order to undergo PGT (preimplantation genetic testing). Many people elect to freeze embryos for multiple chances at successful pregnancies.
Embryos are frozen through a process known as vitrification, or rapid freezing. The embryos are moved through a series of media drops that pull water from the embryo (so ice crystals don’t form) and protect the embryo when it is frozen. Each frozen embryo is then placed on a small, labeled device (see picture) and quickly plunged into liquid nitrogen. Embryos can remain frozen indefinitely inside devices called cryotanks or dewars, which are monitored and regularly filled with liquid nitrogen.
Once embryos are frozen, they need to be thawed in order to be transferred. For an embryo to thaw, it is quickly removed from the liquid nitrogen that it has been frozen in and then moved through a series of media drops. The media causes water to enter the embryo and safely brings the embryo to a warmer temperature. The embryo is then placed into culture fluid in an incubator, where it re-expands and resumes growing as if it was never frozen at all. Embryos are typically thawed a few hours prior to a scheduled transfer to ensure that the embryo survived the thaw and is continuing to properly develop.
Note: the same procedures apply to frozen and thawed eggs.