Embryo Freezing

This is the process of freezing embryos to preserve them over time. Frozen embryos have been successfully thawed and used in later cycles that resulted in pregnancy.

During a fresh stimulated IVF cycle 3-4 embryos are transferred to the patient and the remaining embryos are assessed to determine whether they will be frozen for later use.

The embryo freezing process takes approximately 3 hours. Embryos are sequentially treated with varying concentrations of a special solution, called the cryoprotectant. The cryoprotectant protects the embryos during the freezing process. Each embryo is then drawn into a specially designed sterile straw that is labelled with the patient’s full name and the date of the freeze. The straws containing the embryos are placed into a freezing machine, slowly cooled to -35 degrees C and stored in liquid nitrogen for long-term storage. (-196 degrees C).

Embryo Thawing

Embryo Thawing is the reverse of the freezing process, and involves warming the embryos. Scientists will thaw the embryos either the day before or on the day of the scheduled embryo transfer. Embryo thawing takes approximately 2 hours. Once the embryos are returned to room temperature, the embryos are again passed through a series of solutions to remove the cryoprotectant that is no longer needed. The thawed embryos are kept in the incubator until the embryo transfer, during which time they resume development and may undergo more cell division.

It is possible that some cells within the embryo may not survive freezing and thawing, however the embryo may still be viable. It is also possible that not all of your frozen embryos will survive the freezing and thawing process and in some cases no embryos survive freezing and thawing. There are currently no clear indicators to predict embryo survival.

An obvious benefit is the lower cost since ovarian stimulation, egg retrieval, and fertilization procedures do not need to be repeated with the subsequent cycle.

Frozen Embryo Transfer

The timing of the embryo transfer depends on a number of factors including the cell stage of the embryo when frozen and uterine receptivity. The overall success rates associated with transferring frozen-thawed embryos are, however, less when compared to a fresh embryo cycle.