Frozen Embryo Transfer
At our Northern California San Francisco Bay Area fertility center, we typically freeze embryos five or six days after an egg retrieval procedure, when they have reached the blastocyst stage. We can actually freeze embryos on any day after fertilization has occurred, but we normally wait until preimplantation development is complete so that we know that the embryo has made it through all the preceding stages successfully and is still developing well. The day we freeze embryos depends on the individual circumstances of a particular patient, how well embryos are developing in the laboratory, how many embryos we have, and when a patient is having her embryo transfer. About 60% of patients have a transfer on the fifth day after retrieval and we freeze surplus healthy embryos the same day, and some embryos the next day (day 6) if more have reached the blastocyst stage. At this time, we can see how well the embryos are developing and choose the best embryos for transfer and freezing. Embryos tolerate freezing relatively well on day 5 or 6 and about 50% of patients become pregnant after a transfer using thawed embryos (see table below showing % clinical pregnancies per FET for 2010 and 2011).
|2010 (%) n=311||48||48||41||2/12||36|
|2011 (%) n=369||55||53||49||7/13||49|
After a day 5 transfer, surplus blastocysts are to be frozen for later use. They can be frozen on day 5, or if they are developing a little more slowly, on day 6. Blastocysts have many more cells (up to 200 cells) than day 3 embryos (up to 12 cells) but they freeze just as well. All freezing techniques involve dehydrating the embryo, and specifically with blastocysts, the fluid-filled cavity (cyst) must be collapsed (i.e. the fluid evacuated) before the embryo is frozen (see figure below). When thawed and placed inside the incubator in the laboratory, the cavity will begin to re-expand and the blastocyst should be fully inflated about 2 hours later.
The technology that allows us to grow embryos to day 5 or 6 continues to improve, and in line with this, we are offering blastocyst transfer and freezing to more patients. Blastocyst transfer has the specific advantage of giving us the ability to better choose viable embryos from among a group, so patients that have more embryos than they want to transfer on Day 3, usually culture the embryos on to Day 5 and then pick the best blastocyst(s) for transfer. Individuals using donor oocytes almost always do a day 5 transfer, since we tend to have many embryos to choose from in these cases. Blastocyst transfer will not be an option for everybody, and not everyone will have enough blastocysts to transfer and to freeze. We freeze blastocysts every day in our lab and transfers with thawed blastocysts are a regular part of our laboratory routine. If one thinks they might be a candidate for a blastocyst transfer, they should talk to their physician for more information.
Image 1: A small hole is made between 2 outer cells (5 o’ clock position in this picture) to allow the fluid in the blastocyst cavity to escape
Image 2: The blastocyst collapses in on itself and towards the hole
Image 3: When the embryo is fully collapsed, it is ready to be frozen. After thawing, the embryo will re-inflate in about 2 hours