Embryo transfer (or blastocyst transfer) is one of the most important aspects in the IVF process. After the delicate work by the experienced embryologist to maintain the embryos, the next step is to put the embryos back into the uterus in hopes of a pregnancy. Deciding how many embryos to transfer is a decision made with the doctor and patient, weighing all the factors such as embryo quality, age, and potential risks. In some cases the decision is straight forward, and in others it may be more complex. If good quality embryos remain after the initial embryo transfer, they can be frozen for later use. This freezing process is called cryopreservation.
Performing this procedure at a particular time in the development of the embryo is where the term “blastocyst transfer” comes from. The timing of transferring the embryo is important to the patient’s IVF outcome. In general, embryo transfer (ET) is performed around day three after fertilization. Around five or six days after fertilization, embryos are considered to be at the blastocyst stage of development; there has been a increasing evidence transferring at that stage of development results in higher implantation rates per embryo.
Effectively, blastocyst transfer allows for increased implantation potential with the transfer of fewer embryos. For infertility patients where the goal is a single healthy baby (not a multiple pregnancy) this result is especially important, because it lowers the chances of dealing with potential health risks of multiple pregnancy.
An embryo can be frozen when it is transferred during an initial IVF cycle and deemed high enough quality for cryopreservation, or freezing. It is important to remember that not all cycles have embryos that are suitable for freezing. The rate of survival following thawing of frozen embryos is approximately 70%, so only good quality embryos should be frozen.
Embryo thawing is basically a reversal of the freezing procedure. After the embryos are removed from the storage tank, they are warmed to room temperature. Four different solutions are used to remove the cryoprotectant—the chemical used in the freezing solution to protect the embryos—is restore all the water to the embryos. This thawing procedure is performed at room temperature, then the completed the embryos are warmed to body temperature (370oC). Thawed embryos are ready for transfer in as little as 40 minutes after their removal from storage. Then a normal embryo transfer is performed.