IVF cycles often yield more embryos than required or considered safe for one transfer. When the extra embryos develop well during culture, they may be preserved for future use through cryopreservation (or freezing). Cryopreservation requires prior informed consent of the patient. Embryos can be cryopreserved at various stages of development and they can be transferred to the uterus after thaw, at an optimal time in a future cycle. In our program, embryos are usually frozen on Day 5 or 6, that is, at the blastocyst stage, using vitrification or ultra-rapid cooling. Occasionally, embryos are cryopreserved during the cleavage stages on Day 3 of development, using a conventional slow-cooling procedure. Embryos can be cryopreserved at earlier stages but our program does not use those protocols. We do not cryopreserve every extra embryo, but only those that show good development during the first three days in culture and those that form good quality blastocysts by day 5 or 6 in culture. This policy aims to optimize the patient’s chances of success during a thaw cycle while avoiding unnecessary added expenses and the disappointment of a failed thaw cycle. More than 95% of the embryos we freeze and thaw survive these procedures. Cryopreservation criteria at the blastocyst stage include the following: blastocyst stage must be reached by Day 5 or 6 in culture and the blastocyst must have a defined inner cell mass and an intact trophectoderm. Prior to 2007, our program used a slow-cooling method to cryopreserve blastocysts.
Starting in 2007, we began to use a method called S3-vitrification, developed by one of our scientists at the ART Institute of Washington to cryopreserve embryos at this developmental stage. We prefer this method to others because it produces very high post-thaw survival rates and may be safer. The S3 method uses straws for storage of embryos. In almost all cases, each straw contains one blastocyst. Each straw is labeled with patient identifiers including the female patient’s name and the sponsor’s last 4. Once the embryo(s) are cryopreserved they are transferred to a cane with a unique number as an additional patient identifier and this number is noted in both a physical logbook and an electronic spreadsheet. This cane is stored in one of our special liquid nitrogen tanks to maintain the cryopreserved embryo(s) at -196°C (-346°F) until a thaw cycle is scheduled. Transfer of a cryopreserved and thawed embryo is similar to that of a fresh embryo.