On the day of embryo transfer, a couple may learn that they have additional embryos of good quality in addition to those embryos that have been selected for embryo transfer. These embryos can be cryopreserved by freezing them in liquid nitrogen. Through a series of carefully orchestrated steps, the embryos are ultimately frozen at a temperature of –196 C, leaving them in a state of suspended animation in which they can remain for many years. Embryos that have been stored for more than 10 years have successfully generated pregnancies (although most patients tend to use their frozen embryos within 3 to 5 years after they are produced). The pregnancy rates associated with replacing frozen embryos depend on the age of the patient and the quality of the embryos at the time of cryopreservation. Top-quality embryos from young patients may yield pregnancy rates in excess of 50%, whereas poor-quality embryos may not even survive the thawing process. In some clinics, more than 75% of embryos survive the freeze–thaw cycle. Currently, two methods are used to freeze embryos: "slow cooling" and vitrification.
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