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Since most IVF programmes 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 3 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 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 -196º 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.

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.

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