The cryopreservation or freezing of embryos is the process by which remaining, good quality embryos from a given IVF cycle could be stored for future use. Cryopreservation of embryos allows for the possibility of pregnancy either when a fresh cycle is unsuccessful or when couples want additional children after a successful embryo transfer. Cryopreservation also avoids many ethical dilemmas by eliminating the need to dispose of embryos. In addition, the medical literature has reported that there is no increase in birth defects or developmental abnormalities in children born from frozen embryos.
As with the case of cryopreserved sperm, many embryos do not survive the freezing and thawing process. In fact, approximately 30-40% of embryos survive the thawing process. Embryos that do survive the thawing process may function less optimal than do fresh embryos, in the sense that they implant and produce ongoing pregnancies at a lower rate than fresh embryos.
Embryo morphology (appearance of the cells/ percentage of fragmentation) is one of the most important factors determining embryo survival. Embryos that are frozen at the 2 through the 8 cells stage of development have about 5-10% greater survival than embryos with an odd number of cells. For this reason, in order to optimize successful pregnancies in the future, only embryos of good quality are candidates for cryopreservation,
A computer-controlled machine that employs special solutions to protect the embryos from damage allows the cryopreservation process to take place. Nowadays, the process called vitrification is the freezing technique of choice in order to obtain more embryos that are viable. This freezing technique is especially important for embryos that are frozen at the blastocyst stage. Blastocysts are much larger embryos that require special handling in order to avoid damage during the freezing process. Frozen embryos are stored in liquid nitrogen at -196°C (approximately -400°F).
Cryopreserved embryos that successfully survive the thawing process are usually transferred back to the uterus during an artificial menstrual cycle in which the patient takes leuprolide, estrogen and progesterone. Clinical pregnancies have been reported from embryos that have been stored as long as 10 years.
Harvesting and freezing of mature eggs, which could later be fertilized with a partner/donor sperm at a later time, avoids the issues of a patient having to have a partner or pick a sperm donor immediately and prevents the formation of “orphan embryos”. The first baby from a frozen egg was born in 1986. Since then, there have been more than 1,000 babies born worldwide using this technique.
Freezing eggs is more complicated than freezing sperm or embryos. The egg is a large cell which contains a lot of water in its cytoplasm. The water inside of the egg can form ice crystals which can damage the egg and its chromosomes. In order to avoid this damage, a new technique called vitrification has been implemented lately with very encouraging results.
Because the birth rate from frozen eggs is lower than that of conventional IVF, the American Society for Reproductive Medicine (ASRM) considers this technology experimental. Nonetheless, ASRM considers egg freezing an appropriate option for cancer patients, as long as they are informed about the potential risks from the fertility drugs and egg retrieval, the costs associated with the technique and the lower success rates.
Ovarian tissue freezing
Many oncologists do not support the use of assisted reproductive technologies because it involves the exposure of a cancer patient to high levels of estrogen, which could be detrimental in women who have estrogen-dependent tumors. For these women, ovarian tissue freezing may be an option. This therapeutic modality does not require hormonal stimulation and may be used in pre-pubertal patients. Many immature eggs may be preserved this way, but extensive laboratory culture and/or surgery to replace the ovarian tissue back in the body is still necessary. Even though “auto-grafting” has resulted in a few pregnancies, this therapy is still in early development.
Although these technologies of fertility preservation are available, it is important to notice that there are no large randomized clinical trials to evaluate these interventions. There are no long-term follow up studies assessing the possible effect that these techniques may have on cancer survivors. To date, the available medical literature has demonstrated that pregnancy after cancer treatment does not trigger cancer recurrence, even in survivors of breast cancer. While chemotherapy can damage growing eggs and sperm, this damage generally disappears within 6 months to 2 years after therapy. Nonetheless, pregnancy after cancer can have some increased risks, such as maternal heart failure after certain types of chemotherapy. These pregnancies must be closely followed by a specialist in maternal fetal medicine.
In addition, birth defects in children born to cancer survivors are not any higher than in the general population (2-3%). No unusual risk for genetic diseases, such as Down syndrome and Turner syndrome, has been identified in the offspring of cancer survivors, neither has the risk of unusual cancers been identified in these children. Patients with inheritable diseases can have treatment with in vitro fertilization and pre-implantation genetic diagnosis to select for unaffected embryos for implantation into the uterus.