The main goal of embryo freezing is to cryopreserve the embryo for later use. The first successful pregnancy that resulted from freezing a woman’s embryos was in the early 1980s. Since then, the factors in the IVF laboratory, including culture media, the ability to grow embryos in vitro to the blastocyst stage at day 5 or day 6 of embryo development, and vitrification (a “fast-freeze” process, which has effectively replaced “slow-freezing” of embryos in the majority of IVF laboratories) have led to significantly improved frozen-thawed embryo transfer (FET) live birth rates which rival, and in certain clinical scenarios may exceed, fresh embryo transfer success.
When a fresh embryo transfer is performed, surplus blastocyst cryopreservation essentially provides opportunity for future pregnancy attempts with FET as an extension of the initial egg retrieval. Due to the improved outcomes with embryo freezing, the freeze-all option is being used more commonly. The most common medical indications for the freeze-all option include cases of very high peak estradiol associated with excess follicular growth (which would increase the risk of severe ovarian hyperstimulation syndrome and potentially decrease pregnancy rates due to an endometrial environment that is not hospitable to implantation if the patient undergoes fresh embryo transfer that cycle) and/or elevated progesterone levels (which can advance the endometrium too quickly, essentially decreasing pregnancy rates due to asynchrony of embryo and endometrium) on the day of hCG trigger prior to oocyte retrieval, which would temporarily adversely affect the endometrial lining during the oocyte retrieval cycle. In such cases, FET provides a better chance for pregnancy due to a more natural endometrial environment. In addition, freeze-all of embryos is being performed more commonly due to the increase in number of cases of preimplantation genetic screening/preimplantation genetic diagnosis (PGS/PGD) while results of genetic tests are pending after some cells are biopsied from blastocyst stage embryos. However, convincing published data from well-designed studies are necessary prior to implementation of routine freeze-all for all IVF cases.