Similar to oocyte freezing, embryos and blastocysts are preserved at sub-zero temperatures. The embryonic stage of development corresponds to the pre-implantation, where under normal circumstances the fertilized egg is still within the fallopian tube en route to the uterus, where it would develop into a fetus.
After oocytes are harvested, they are fertilized using donor sperm and allowed to develop into embryos over three days. Embryo cryopreservation can be achieved via slow freezing or vitrification, much like what is achieved when oocytes are frozen.
Embryo cryopreservation is indicated for “leftover” embryos after the completion of an IVF cycle. During a single IVF cycle, many oocytes can be harvested and fertilized, but only a few are transferred back to the uterus. If the transfer results in a failed pregnancy, the other cryopreserved embryos can be transferred in a subsequent attempt. In the case of a successful transfer, the embryo can be transferred to the mother’s uterus in the event she is ready for another pregnancy.
Embryos that have been created in the fertility treatment process may also be used for donation to another woman or couple trying to become pregnant.
The success rates for the transfer of cryopreserved embryos have been reported to be similar to fresh (i.e. non-frozen) embryos, with no increase in birth defects or other abnormalities. Also, storage time has been shown to have negligible effects on the embryo survival rates, implantation, pregnancy, or live birth rates.
A blastocyst is the subsequent developmental stage wherein the embryo is between five and seven days old. Compared to embryos, blastocysts are further along in their development, and are less vulnerable to some of the developmental pitfalls that may harm embryos. Additionally, since embryos are allowed two to four days to progress, the less viable embryos tend to fail, leaving behind their stronger cohorts, which are more likely to succeed. Embryos that fail to make it to the blastocyst stage usually have a chromosomal abnormality at fault.
Several studies have determined that blastocysts tend to have a better survival rate after cryopreservation than do embryos or earlier stages of development. Thus, blastocyst freezing has been considered to be superior to embryo cryopreservation.
The ability to thaw and transfer blastocysts on the same day is another advantage of blastocyst freezing that is not present in embryo cryopreservation. Embryos that have been thawed often must be allowed to develop into blastocysts prior to transfer into the uterus.
Similar to embryo cryopreservation, both slow freezing and rapid vitrification processes cam be performed, with vitrification appearing to be the successful, leading to higher survival and pregnancy rates. Despite similar indications for blastocyst cryopreservation and embryo cryopreservation, many fertility clinics prefer cryopreservation of blastocysts due to their superior viability and higher success rates.