Cryopreservation or freezing offers a way to store embryos for a subsequent embryo transfer cycle. Couples who have extra embryos available after an IVF procedure need to decide what to do with them. The extra embryos may be frozen, or cryopreserved, after fertilization.

In the event excess good quality embryos are created during an IVF egg retrieval, cryopreservation will be offered. Cryopreservation is the freezing and preservation of embryos in liquid nitrogen at minus 190ºC for the purpose of using them in a future transfer cycle. Only good quality embryos will be frozen for future use. As part of a future IVF transfer called a frozen embryo transfer (FET), instead of retrieving eggs the cryopreserved embryos will be thawed for transfer and implanted in the uterus. There is no guarantee that the embryos will survive the freezing and thawing process or that, if FET occurs, pregnancy will result.

Since cryopreserved embryos are frozen in groups of one or two, we can individualize the number of embryos transferred in an FET cycle. A decision on how many embryos to transfer is predetermined prior to the thawing of the embryos. Only embryos considered to be potentially viable using reasonable medical judgment will be transferred.

Frozen embryos can be stored for five years. Typically, 10-20% of the embryos do not survive the freezing and thawing process. Pregnancy rates with frozen embryos have been shown to be similar to nonfrozen embryos of equal quality. However, sometimes the best quality embryos are transferred to the uterus in the fresh IVF cycle and the excess embryos frozen are the "second" best in quality. That is why FET may be associated with a somewhat lower pregnancy rate.

Cryopreservation of embryos has several benefits. During a frozen embryo transfer cycle, the woman does not need to use gonadotropins to stimulate her follicles. A woman will use hormones in the form of estrogen and progesterone medication. The FET cycle is less physically demanding, and the cost of a frozen embryo transfer cycle is much less than the cost of a regular IVF cycle.

Remember, not all patients will have excess embryos to freeze. Approximately 75% of women under the age of 35 with a day three FSH <7 mIU/ml and day three estradiol <75 pg/ml will have extra embryos to freeze. Only 10% of women over 35 or with a Day 3 FSH >8 or a Day 3 estradiol >75 pg/ml will have extra embryos to freeze. Couples with severe male factor infertility tend to have fewer embryos created (lower fertilization rates) and hence are less likely to have embryos to freeze.

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