Techniques of controlled-rate freezing are utilized that slowly cool embryos in cryoprotectant fluid ("anti-freeze" solution) from body temperature down to -196°C, at which temperature they are stored in containers of liquid nitrogen called dewars. The embryos are actually contained within special indelibly labeled plastic vials, or straws, that are sealed prior to freezing. Once frozen, they are placed inside labeled tubes attached to aluminum canes and stored in numbered canisters within the liquid nitrogen dewar. Site and label designations are stored in three separate file systems to avoid confusion and misidentification of cryopreserved embryos. 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 at room temperature, which takes about one to two minutes. However, the most critical 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. Experience has shown that if the embryo survives 50% or more intact, it is worthwhile to replace it. Embryos can accommodate such levels of cellular damage and still establish healthy pregnancies. All thawed embryos 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 the outer embryo shell.

Varying strategies may be applied according to how many and which embryos are thawed prior to transfer. It should be noted that not every couple undergoing IVF will need to worry about embryo freezing/thawing, since not every couple will have sufficiently large number of "surplus" or non-transferred embryos available for freezing. Indeed, most couples have only one or two embryos frozen, so that all are thawed and any surviving are replaced. In the event that there are more than two or three embryos frozen, thawing is usually undertaken until two to three healthy appearing embryos are recovered. In some cases, this may mean that all the cryopreserved embryos are thawed, in others just two or three. There always remains a possibility that there may be no embryo survival after thaw occurs, and no transfer is possible. If many early embryos are frozen, it is possible to thaw all of them and culture them for several days to allow selection of the best for transfer. When too many embryos are available for transfer in this circumstance, then extra embryos of sufficient quality may be refrozen for later use. This course of action has produced healthy offspring, proving the efficacy of double freezing of embryos.

During a medication-prepared frozen/thawed embryo transfer cycle as a patient, you will follow a treatment schedule using Synarel or Lupron, estrogen (pills, lozenge or patch) and progesterone (lozenge and/or suppository) in order to achieve appropriate endometrium (uterine wall lining) for embryo transfer. Following embryo transfer, estrogen and progesterone will be administered daily until the 7th to 8th week of pregnancy or until a negative pregnancy test.