embryo freezing and storage
Embryo freezing (cryopreservation) is a technical procedure by means of which embryos are stored for long periods of time at a low temperature (-196oC). The period from fertilization to day 14 is called "embryonic" and therefore the term embryo is used. Embryos can be frozen between the single cell stage and the blastocyst (32-64 cells) stage of development. Water in the embryos is replaced with a chemical solution (cryoprotectant) that functions like antifreeze. If a cryoprotectant is not used, as the temperature decreases, water in the embryos freezes and forms ice crystals. These crystals destroy embryos. When embryos are thawed, the cryoprotectant is removed and replaced with water.
Typically, either two or three embryos are transferred to the uterus during an IVF cycle. However, some cycles will produce more than two or three embryos. Embryo freezing provides a way that these extra or "spare" embryos can be stored and returned during a later cycle. Embryo freezing, therefore, increases the options available to IVF couples.
Prior to freezing, the quality of the embryos is evaluated based on appearance to determine their suitability for freezing. Studies suggest that embryos that show physical blemishes of 25% or less are more likely to survive the freezing procedure. Consequently, even when there are spare embryos, they may not be suitable for freezing because of their poor quality.
HOW WILL FROZEN EMBRYOS BE RETURNED?
Frozen embryos usually are returned during a natural menstrual cycle (no medications). During this cycle certain hormone levels are measured daily. In some cycles medications may be given to prepare the lining of the uterus for implantation. At the appropriate time, the frozen embryos are thawed. A maximum of three frozen embryos will be thawed at any one time until a maximum of three embryos are suitable for embryo transfer. The embryos are then cultured overnight to determine whether they have survived the procedure. The day after thawing, the surviving embryos are transferred using the same method as that used to transfer embryos during a stimulated IVF cycle. The timing of thawing and transfer is determined on an individual basis.
Please note: after embryos are frozen, no further stimulated IVF cycles will be undertaken until all frozen embryos have been reclaimed, donated for research purposes, or discarded.