Cryopreservation of human embryos has been a routine procedure since the early 1980s. Conventional techniques (which are still being widely used by most IVF centers) involve a slow freezing process that attempts to prevent water content in the cells from forming ice crystals. When crystals form, embryos become damaged and lose their viability. Since 2006, we have instead used an ultra-rapid freezing process called vitrification. This innovative process rapidly freezes the cells, thereby preventing intracellular ice formation and avoiding cell damage. Vitrification involves freezing embryos or eggs in a tiny amount (less than 0.1 microliters) of a special solution, before storing them in liquid nitrogen. Conventional slow freezing yields a survival rate for human embryos of about 75-80%. By contrast, vitrification improves post-thaw embryo survival to >90%.
It is our preference to cryopreserve only embryos that make it to the blastocyst stage. This is based upon our own recent research, published in Fertility and Sterility in 2007, which demonstrated that embryos failing to develop to the blastocyst stage are virtually always chromosomally abnormal (aneuploid). Many In Vitro Fertilitzation programs still freeze embryos on day 2 or 3 after fertilization. Your physician should discuss this choice with you so that your needs and goals are met. Many physicians agree with this position of exclusively freezing only “good quality” (grade 1 or 2) blastocysts (120-144 hours post egg retrieval) while others still encourage day-3 or earlier embryo freezing.
Since it takes several weeks to optimally perform complete CGH testing on eggs/embryos or blastocysts, it is not possible to have the test result available in the time for a transfer of a fresh blastocyst in the same cycle. By comparison, the partial genetic testing of PGD requires 2-3 days following the biopsy, which allows fresh embryos to be transferred in the same IVF cycle. Thus it is currently necessary to cryopreserve (vitrify) and store the blastocyst(s) for subsequent FET in an upcoming menstrual cycle (i.e. Staggered IVF). The safety and efficacy of the vitrification method now makes it possible to transfer thawed blastocysts by FET with minimal concern for damaging an otherwise healthy embryo.
While the treating RE generally recommends when the embryos should be frozen, the final decision is really up to the patient/couple. The RE should guide the process to best meet their expectations. At most of our centers, thawed embryos are first allowed to develop to the blastocyst stage before being transferred to the uterus – regardless of the stage at which the embryos were frozen. This means that when eggs have been frozen at the earliest stage of development (2PN or pronucleate eggs), they would be thawed and cultured for several days. Those that attain the blastocyst stage of development are eligible for transfer to the uterus. By contrast, frozen blastocysts are thawed and then transferred a few hours later. All available evidence suggests that the replacement of thawed blastocysts does not increase the risk of birth defects.