Embryo freezing is an important part of the IVF process. Patients who have additional good quality embryos can freeze them for future use. These embryos provide a second or even a third opportunity for pregnancy without undergoing another ovarian stimulation and retrieval.

Embryos that meet developmental criteria for appearance and rate of growth can be frozen at any of several stages of development.

One is called slow cooling. With this method embryos are placed into special freezing solutions, and using a computer, the temperature of the embryos is slowly decreased. Frozen embryos are then stored in liquid nitrogen (at -196°C or approximately -400°F), or sometimes, in liquid nitrogen vapor.

Another technique for freezing embryos is called vitrification. In this ultra-rapid freezing method, embryos are placed into special solutions and then placed immediately into liquid nitrogen. Embryos are stored same as in slow cooling. The method used to freeze embryos dictates how the embryos must be warmed or thawed. Not all embryos survive the freezing/thawing procedure and sometimes an embryo cannot be found after freezing.

A Danish study published at ESHRE showed that there is concern about congenital malformations in babies born after thawing of freezed embryos. Moreover the same study proposed that maybe all embryos should be freezed and not transfered given that the conditions under which a fresh transfert occurs are not so normal due to high maternal hormone levels during stimulation.