In this webinar, Karen Schnauffer, Consultant Embryologist & Laboratory Director at RHG – IVF Life Group, UK explained how freezing your embryos works and provided an interesting Q&A session.
Karen Schnauffer, the HFEA person responsible, discussed how the embryos are frozen and started by explaining that it is possible to freeze many stages of embryos. We can freeze eggs, fertilised eggs and then when the embryo starts dividing into 2-4 cells, then it is 8 cells and finally, blastocyst stage and hatching blastocyst. The survival rates are much better nowadays with a process called vitrification, and that works best with the blastocyst stage.
What we’re looking for when we’re freezing blastocysts is a little structure in the middle called the inner cell mass surrounded by lots of little cells called the trophectoderm, and then the whole embryo is full of a fluid cavity. There are various stages and various types of quality. There are different levels we can freeze and some poorer qualities that we can’t. When we have a fully expanded mature blastocyst, we do a grading based on the inner cell mass and trophectoderm.
For example, a blastocyst that has moved on to the following stage where it started to hatch out of the shell called the zona pellucida, and this surrounds the egg and then the embryo throughout its development until it finds a weak point and hatches out and that’s when it is ready to implant. A fully hatched blastocyst doesn’t have a shell surrounding it, and it’s surrounded by inner cell mast cells.
There are various devices for freezing and different culture media. Dr Schnauffer concentrated on what she uses at her lab. There is a straw called rapid eye, it’s manufactured by one of the IVF companies, and it has an outer and an inner where the embryo sits, and there’s a tiny hole, and the blastocyst is placed just inside the hole. The whole thing is then placed into the outer sheath, and then it goes to freezing. There’s also a box where the straws sit submerged in liquid nitrogen so that the straw is at the right temperature throughout that process.
We use a culture dish that has the different solutions for freezing the blastocyst, and they go into each of the different solutions for a certain period. Then we move them to the next one, and we wait for the period it needs to be in that solution, and then once it’s gone through this process, which takes about 12 minutes, we load the embryo up. Then, we use a little glass pipette, and the blastocyst is placed into that. As soon as that’s done, we have to place it into the outer straw which has been submerged in liquid nitrogen. We heat, seal it, the bottom is already sealed and then it is plunged straight into liquid nitrogen, and it’s sealed and frozen.
After that, the embryos need to be labelled. We have to make sure that we can identify them, and that is done by labels. The labels used in freezing are wrap-around labels, so they’re completely secure, nothing can be scraped off, and they’re also temperature resistant. Most freezing programs also use a lot of colour coding, so it’s very easy. A lot of systems nowadays use a barcode reader as well, so not only you’ve got the details printed on the label, but you’ve also got a barcode to confirm the details on whatever system you’re using.
From the moment the embryos are frozen and put into the straws and placed in a special container, which contains liquid nitrogen, they are then moved to the actual storage. A storage vessel is filled with liquid nitrogen at -196 degrees, so the embryos are suspended in time. From then on, they are frozen until they are thawed.
Based on the UK information, a lot of people are probably quite concerned if they don’t have a fresh transfer, they feel that maybe they’re not giving themselves the best chance, but when you have your fertility treatment, sometimes you’re not able to have the fresh transfer. It might be that the lining is not appropriate, the environment is not appropriate, there’s been some overstimulation, and it’s not safe to proceed, etc. Therefore, the process would be that the embryos are frozen, and the patient then recovers and comes back for a future date to have her frozen embryos thawed and transferred.
The Human Fertilization Embryology Authority’s (HFEA) most recent data showed the birth rates for fresh and frozen transfers. Since 1991, when they started recording this information, the pregnancy rates were very low, it was 5-8%, and they’ve steadily got higher and higher until 2017, which is the latest data released by HFEA, and the live birth rates are around 22%. If we have a look at birth rates with frozen embryos, they did lag for many years behind the fresh embryos, so it was beneficial back then to have a fresh embryo transfer. Around 2011 and onwards, when the vitrification process became the normal way of freezing embryos, the birth rates with frozen embryos got higher than the fresh ones. It’s not that your chances are better if you have frozen embryos, but that your chances of pregnancy are not decreased if your embryos are frozen.