Cryopreservation, also known as "freezing," involves storing embryos at a very low temperature so that they can be thawed and used later. Many fertility clinics now offer patients this option.
There are many reasons for using cryopreservation. It is often done when there are more embryos than are needed for a single in vitro fertilization (IVF) cycle. The extra embryos are saved and potentially used during later cycles.
IVF success rates are generally lower with frozen embryos than with freshly fertilized embryos, although pregnancy rates with frozen embryos have been improving in recent years. The survival rate after thawing often depends on the quality of the embryos at the time of freezing.
Since most IVF programs superovulate patients to grow many eggs, there are often many embryos. Since the risk of multiple pregnancies increases with the number of embryos transferred (and in fact the law in the UK prohibits the transfer of more than 2 embryos to reduce this risk), many patients are left with "spare" or supernumerary embryos. These can be discarded; or used for research.
It is now also possible to freeze these embryos and store them in liquid nitrogen. These stored embryos can then be used later for the same patient - so that she can have another embryo transfer cycle done without having to go through superovulation and egg collection all over again. Moreover, since this embryo transfer is done in a "natural" cycle ( when she is not taking any hormone injections ) some doctors believe the receptivity of the uterus to the embryos is better. For women with irregular menstrual cycles, frozen embryo transfer can also be done in a " simulated natural cycle", in which the endometrium is primed to maximize its receptivity to the embryos by using exogenous estrogens and progesterone.
Since pregnancy rates with good-quality frozen-thawed embryos are as good as with fresh embryos, we encourage all our patients to freeze and store their supernumerary embryos, rather than discard them. Freezing is very cost-effective, since transferring frozen-thawed embryos is much less expensive than starting a new cycle, so that it serves as a useful "insurance policy" in case pregnancy does not occur. However, since it is worthwhile freezing only good quality embryos, the option of freezing is a "bonus" which is available to only about 30% of all IVF patients.
About half of all embryos frozen survive the freezing -thaw process. It is reassuring to know that the risk of defects is not increased as a result of freezing. These frozen embryos can be stored for as long as is needed - even for many years. When they are in liquid nitrogen, at a temperature of -196 C, they are in a state of suspended animation, and all metabolic activity at this low temperature stops, so that a frozen embryo is like Sleeping Beauty !
Once stored, embryos can be used by the couple during a later treatment cycle, donated to another couple or removed from storage. These options should only be undertaken after considerable discussion and written consent from the parties concerned.
Egg and ovarian tissue freezing are experimental techniques being investigated for fertility preservation in women. These techniques have the potential to be helpful for women with cancer, as chemotherapy and radiation treatment can often have a harmful effect on fertility. In egg freezing, a woman's mature eggs are developed and removed using standard IVF techniques. They are then frozen before being fertilized with sperm, in the hopes that they can be thawed, fertilized and transferred back into the woman's uterus later.
In ovarian tissue freezing, the ovary or ovaries are removed laparoscopically, divided into small strips, then frozen and stored. Later, they may be thawed for transplantation back into a woman's body, or for use with in vitro maturation, an investigational technique in which eggs are matured in the laboratory, rather than in a woman's body.
While we still cannot freeze unfertilised human oocytes efficiently, a new technique called vitrification ( which uses ultra-rapid cooling together with an increased concentration of cryoprotectants ) may allow us to offer this option to our patients, in the future, allowing the facility of egg storage and egg banking.
It's important to remember that both egg and ovarian tissue freezing are experimental techniques. The American Society for Reproductive Medicine (ASRM), has recommended that these techniques be studied further before they are widely used.
