We typically freeze embryos one, three or five days after an egg retrieval procedure. 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 their embryo transfer. Most patients have a transfer on the third day after retrieval and we freeze surplus healthy embryos the same day. 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 3 and about one third of patients become pregnant after a transfer using thawed embryos.

It is not common to freeze embryos on day 1 after retrieval since at this time we have very limited information on their development, but this is the stage at which embryos best tolerate freezing and thawing. In 2005, 92% of embryos thawed at this stage survived, compared to 64% of embryos surviving after freezing on day 3. However, the lack of embryo development information on day 1 means that we are probably freezing many embryos that have little or no chance of establishing a pregnancy. We therefore prefer to let the embryos grow for at least another 2 days to make sure that we only end up with good quality embryos in our freezer.

In the last 2 years we have been doing more and more transfers on day 5-post retrieval (blastocyst transfer). Delaying the transfer an additional 2 days allows us to get a much better picture of which embryos in a cohort are really strong and healthy. By day 5, the embryo should have reached the blastocyst stage, which is characterized by the presence of a fluid filled cavity or cyst in the embryo. Embryos that reach the blastocyst stage by day 5 have a higher chance of implanting after transfer when compared to embryos transferred on day 3. However, not all embryos that look healthy and strong on day 3 will make a blastocyst. We estimate that it takes 3-4 nice day 3 embryos to achieve a nice blastocyst on day 5. Therefore, blastocyst transfers are usually undertaken only by patients with many nice embryos on day 3. Also, patients at high risk for a multiple pregnancy and/or those wishing to transfer only one embryo often decide to do their transfer on day 5.

After a day 5 transfer, surplus blastocysts can 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) and we employ a different method for freezing them. All freezing techniques involve dehydrating the embryo, so the fluid filled cavity in a blastocyst will collapse during freezing. 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 4 hours later. Blastocysts that show little or no signs of re-expansion are unlikely to implant after transfer.

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. In particular, our ability to culture embryos in a reduced oxygen environment reduces stress on the embryos and therefore provides healthier embryos for transfer and freezing. In 2005, slightly less than 10% of our fresh and frozen embryo transfers were performed on day 5, but this number is likely to increase dramatically in 2006. Individuals using donor oocytes comprised almost half of the day 5 transfers, 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 freeze. We are freezing blastocysts almost every day now and transfers with thawed blastocysts are becoming a regular part of our laboratory routine. If you think that you might be a candidate for a blastocyst transfer, please talk to your physician for more information.

– Joe Conaghan, PhD, HCLD

Tags: Embryo Freezing, Lab, Treatment Options

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