Embryo Freezing Advantages of Embryo Freezing The cryopreservation procedure provides couples the option of freezing high quality unused embryos for future use. Transfer of thawed embryos is done in cycles with no stimulation drugs or egg retrieval and is less costly. Which Embryos are Cryopreserved Only embryos that develop into good quality blastocysts on day 5 or day 6 post-retrieval will be frozen. Approximately 90% of blastocysts survive the freeze/thaw procedure. Pregnancy rates with thawed blastocysts are very similar to that of fresh blastocysts. Preparation of the Uterine Lining for Thawed Embryo Transfer The uterine lining is prepared for embryo transfer with estrogen patches and progesterone.