Following the successful retrieval and fertilization of a large number of eggs, multiple embryos may be available. Depending on certain maternal and embryonic factors, an optimal number of embryos are implanted in the uterus. Any remaining high-quality embryos may be cryopreserved (frozen) for future implantation. These embryos can be frozen at any stage (pronuclear, early cleaved and blastocyst embryos) if they are of good quality. Embryos are stored in batches of one or more embryos depending on the number of embryos that are likely to be transferred into the uterus at a later date.
There are two methods currently in use for freezing embryos -slow cooling and vitrification. During slow cooling, the embryos are exposed to solutions to help them survive the freezing process, are loaded into small cryostraws then placed in a computer-driven freezing machine that drops the temperature down to -190°C (the temperature at which the straws containing the eggs will be stored). During the vitrification process, embryos are also exposed to solutions that will help them survive the cooling and warming process, are loaded into very small straws called CryoTips and are then plunged directly into liquid nitrogen for ultra-rapid cooling. Once vitrified, the CryoTips are stored at -190°C.
Embryo freezing has been an integral part of assisted conception treatment since IVF became an option. The first baby born after thawing and transferring frozen embryos was in 1984. Therefore, embryo cryopreservation allows multiple embryo transfers from a single egg collection and may improve the chances of cycle success and for live birth.
Advantages of Embryo Cryopreservation
Embryo cryopreservation maximizes the possibility for conception for IVF and prevents discarding viable embryos. It allows us to transfer fewer embryos in a fresh cycle and therefore reduce the risk for multiple gestations. In patients at risk for , freezing all embryos for transfer in a subsequent cycle may be recommended to reduce the risks associated with severe overstimulation. It may also be recommended in cycles when embryo implantation may be compromised in cases such as the presence of endometrial polyps, poor endometrial development, and significant break through bleeding near the time of embryo transfer or patient illness. Rarely, cryopreservation may be necessary when significant difficulty is encountered when navigating the endocervical canal at the time of a fresh embryo transfer e.g. cervical stenosis (inability to pass through the cervical canal because the cervix is narrowed or scarred). Embryo cryopreservation improves the chance of success with oocyte donation cycles. It is also an integral part of the care for patients undergoing the burden of cancer treatment to allow them the opportunity to parent after completing their treatment.
Ethical and Moral Issues Related to Human Embryo Freezing
Although the technology of embryo cryopreservation allows us to provide opportunities for future success, it raises several ethical and moral issues. This may include the concern for the destiny of the stored embryos in the case of changes in the parental relationship due to divorce or death of one or both individuals. There is also concern about the safety of embryo cryopreservation. Although current information suggests that there are no detrimental effects or increase in fetal abnormalities after embryo cryopreservation, there are no long-term studies to confirm these observations.