The first pregnancy from a frozen/thawed human embryo was reported in 1983, and a birth from this source occurred the following year. Of 99,629 cases of Assisted Reproductive Technology (ART)* in the United States in 2000, about 16% of cases (16,194) used frozen/thawed embryos. In 2000, live birth rates per thaw cycle were 18.3% versus 26.6% from fresh embryo transfer. At GRS, the ongoing pregnancy rate for IVF using frozen/thawed embryos is currently 52%.
CRYOPRESERVATION OF HUMAN EMBRYOS
Egg retrieval under ultrasound guidance and subsequent fertilization and embryo culture are carried out according to our current procedures. If there happens to be a surplus of embryos following selection for fresh transfer (usually between one to four embryos are transferred to the uterus), then embryos of sufficient quality may be considered for cryostorage. While embryos can be frozen at any preimplantation stage between one-cell (one day old) to the blastocyst stage (5-6 days old), in an attempt to minimize the freezing of excessive numbers of “spare” embryos and to help pre-select the most potentially viable embryos, we generally choose to cryopreserve only at the blastocyst stage. In certain cases where all embryos need to be frozen without a fresh transfer (e.g., when a woman may be at risk from ovarian hyperstimulation that might be complicated by pregnancy), we generally freeze all embryos the day after egg collection at the one-cell stage.
Techniques of controlled-rate freezing are utilized that slowly cool embryos in cryoprotectant fluid (“anti-freeze” solution) from body temperature down to -196C, 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.
CONSIDERATIONS AND RISKS
The Ethics Committee of the American Society of Reproductive Medicine (ASRM) has published guidelines for ethical consideration of human embryo cryopreservation. Possible advantages of cryopreservation of embryos suggested by the Committee include:
- Reduction of the risk of triplets or quadruplets by cryopreservation of embryos exceeding an optimal number for transfer to an individual patient
- Possibly increasing pregnancy rates by replacing thawed embryos during spontaneous ovulatory cycles or cycles in which the estrogen and progesterone hormone levels do not exceed that which occurs naturally
- Possibly decreasing the number of stimulated ovary drug treatment cycles needed for the attainment of pregnancy
The primary concern with the use of cryopreservation techniques is the possible loss of embryos to cryoinjury, meaning some healthy embryos may not survive the stress of freezing & thawing. The exact number of embryos lost to cryoinjury varies, but it is very likely that freezing will cause loss of some embryos, perhaps as many as 25-50% of those cryostored. One interpretation of this is that cryopreservation may even act as a “selection gate” for the more viable embryos, though this has never been proven.
Another concern with cryopreservation is the potential risk of birth defects in children produced from frozen/thawed embryos. In the domestic animal industry, large-scale freezing and transfer of embryos has not resulted in increased birth defects. Studies to date on those human offspring arising from thawed embryos have not shown any significant increase in abnormalities when compared to pregnancy outcomes in the rest of the population.
To optimize the likelihood of successful embryo cryopreservation at GRS, the mechanical processes of human embryo cryopreservation will be strictly controlled to minimize the chances of technical failure. A back-up freezing system is always available to decrease the risk of interruption in the freezing process, as well as generator back-up power in the event of a power outage. Individual embryos are placed in permanently labeled storage containers and identified according to origin, developmental stage, and date frozen. Permanent records are kept at Georgia Reproductive Specialists for each individual’s embryos. Liquid nitrogen dewars are connected to alarm systems to monitor the liquid nitrogen levels and prevent premature thawing. However, even with all these safeguards, the possibility of technical failure leading to loss of stored embryos following natural disaster cannot be totally and completely eliminated.
PERIOD OF CRYOSTORAGE
The disposition of any frozen embryos that are not transferred must be arranged in writing before cryopreservation. In the event that a successful pregnancy is established following a fresh or subsequent embryo thaw cycle, it will be at the discretion of the couple as to whether the remaining frozen embryos should continue to be cryo-stored, or appropriately discarded or donated. This may include the option to donate the embryos for research or to other infertile couples for transfer.