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. More information on vitrification can be found in Question 72.

Many couples are concerned about their moral obligations concerning their frozen embryos. In such cases, couples may elect to defer embryo freezing, choose to alter their stimulation or pursue Natural Cycle IVF so as to avoid this problem of excess embryos. Extra embryos that are not used to initiate a pregnancy could represent a source of embryonic stem cells. This potential use of extra embryos lies at the heart of the recent political debate in the United States regarding government funding of stem cell research. Clearly, patients should carefully consider the implications of excess frozen embryos as they embark on an IVF cycle. However, not all patients will have extra embryos of high enough quality to be considered for embryo cryopreservation.

Rebecca comments:

I had gone through stim and egg retrieval for my third (and likely last) IVF with my own eggs; egg retrieval occurred the day before my 42nd birthday. I had always responded well to the gonadotropins, and this cycle was no different. I believe my estrogen levels had gone over 10,000 prior to HCG trigger, putting me at risk for developing Ovarian Hyper Stimulation Syndrome (OHSS). On embryo transfer day, I went into the RE’s office feeling very bloated and uncomfortable. I knew embryo transfer was going to be very difficult. My RE immediately suspected OHSS and began discussing the option of embryo cryopreservation to allow for frozen embryo transfer (FET) at a later date. I was heartbroken. I knew that I couldn’t transfer, but was convinced that my ‘older’ embryos would suffer from the freezing process. My RE reassured me that my embryos would be frozen using vitrification and that the clinic was seeing freeze/thaw success rates as high as 85%.

I now believe that cryopreservation and FET were the tickets to our success. The FET cycle was the most relaxed cycle I had been through. There were no shots, there was very limited monitoring and blood draws, and my body wasn’t being overwhelmed by hormones. On the day of transfer, my embryos were thawed with a 100% freeze/ thaw success rate. The transfer of six blasts was the most comfortable transfer of the three transfers I had been through. Two weeks later I received my BFP (positive beta-HCG) and 8 months later I gave birth to my girl/boy twins. I went from being highly skeptical about cryopreservation to becoming a firm believer in the benefits of this wonderful treatment option.