Since multiple eggs (oocytes) are often produced during ovarian stimulation, on occasion there are more embryos available than are considered appropriate for transfer to the uterus. These embryos, if viable, can be frozen for future use. This saves the expense and inconvenience of stimulation to obtain additional eggs in the future. Furthermore, the availability of cryopreservation permits patients to transfer fewer embryos during a fresh cycle, reducing the risk of high-order multiple gestations (triplets or greater). Other possible reasons for cryopreservation of embryos include freezing all embryos in the fresh cycle and postpone the embryo transfer to another time when severe ovarian hyperstimulation syndrome (OHSS) is present, when an individual or a couple were concerned that his/her/their future fertility potential might be reduced due to necessary medical treatment (e.g., cancer therapy or surgery), when there is concern about the suitability of the uterine lining for implantation in the fresh cycle or when results of genetic testing on the embryos are pending.
A cell is made up mostly of water. The main concern when freezing embryos is ice forming in and between the cells. Ice crystals can hurt the cell wall, and can harm the small structures inside the cell.
The embryo must be protected during the freezing process. This is done using special fluids called cryoprotective agents (CPAs). CPAs are like “anti-freeze” for cells.
Doctors use two different methods to freeze and preserve embryos: slow programmable freezing and vitrification.
In the slow freezing method, the embryos are frozen slowly, in stages. The CPAs are added to the embryos in increasing strengths over 10 to 20 minutes. Then the embryos are slowly cooled over two hours in a machine that lowers the temperature minute by minute. Once frozen, the embryos are stored in liquid nitrogen at -321° Fahrenheit (-196.1° Celsius).
Vitrification is a rapid freezing technique that uses much higher strengths of CPAs. With this method, the doctor first mixes the CPAs with the embryos. CPAs that are very strong can also hurt the cell. To prevent this, the embryos are quickly placed into the liquid nitrogen. This process changes them into an almost-solid state, like glass. In this state, ice is unable to form.
When needed, the embryos are slowly thawed. They are soaked in special fluids to remove the CPAs. This also restores the cell’s normal water balance.