IVF Babble

What is Embryo thawing?

There are different reasons you may be choosing to freeze your eggs or embryos

You may have good quality embryos left over after a transfer that you wish to use at a later stage, or may have to put your cycle on hold due to OHSS, with the aim of transferring once your body settles down, or you may just want to freeze your eggs or embryos as a way of preserving your fertility.

According to the HFEA, success rates for IVF  with frozen embryos have been increasing with each year and are now comparable to rates using fresh embryos

However, like everything in life, nothing is ever 100% guaranteed, and if you get the call to say it hasn’t worked, the pain cuts deep, as it did for this reader who reached out to us for answers . . .

“Do you have any information about thawing issues? I had apparently what was the best quality embryo possible, but it didn’t thaw well and quickly diminished. My whole world has come crashing down.”

At a visit to the laboratory at The Lister Fertility Clinic recently, we actually had the honour of watching one of the embryologists carefully thawing an embryo. It was incredible to witness. We were in awe of his skill and steady hands and were keen to ask him the question from our reader about why her embryo didn’t thaw properly.

Does the risk of failure increase the longer an embryo is kept on ice? 

Embryos, eggs, sperm and other human tissue are stored in liquid nitrogen rather than ice and it is the extreme temperature (-210°C to -195°C), combined with the action of the cryoprotectants we use, that allows the cells to tolerate the extreme conditions. 

Studies have shown that at extreme temperatures approaching “absolute zero”, the electrons in free atoms stop moving and lose their connection having no magnetic activity between each other. So, by storing tissue at temperatures near absolute zero we see no quantifiable ageing as there is no biological activity, so the cells are literally frozen in time.

Can you explain what happens during the freezing and thawing process?

Cells are composed predominantly of water and exposing this water to very low temperatures creates ice crystals and such crystals in any cell, can damage irreversibly the structure of the cell itself. In order to prevent that “cryoprotectants” are added to the freezing solutions. 

The freezing process is a very delicate one where we expose it to different concentrations of cryoprotectants over a period of time. During thawing, we remove these cryoprotectants and reintroduce the normal function of the embryo by slowly increasing its cellular water content

Can you explain why some embryos don’t thaw? 

Sometimes despite the fact that the techniques used are very advanced, some embryos might not withstand the process. There are many causes, but the main one is the embryo’s inability to absorb the cryoprotectants during the freezing process. 

What percentage of embryos don’t make the thaw? (Do you always warn your patients that thawing isn’t 100% guaranteed? ) 

The  percentage of survival of embryos varies between  laboratories. Mainly the criteria used to select embryos for freezing will determine a particular laboratory thaw survival rate. Usually top quality embryos are the ones who can withstand the process. Our thaw survival rate is approximately 96% for blastocysts.

Average quality embryos might not survive the process as well as higher quality ones. We would always counsel on the risk of a failed thaw and can thaw a second if available so a cycle isn’t wasted.

If you have any questions on this or other fertility topics, please do ask and we will get the answers from our experts. You can also look out for our live Instagram Q&A sessions. Remember, no question is a silly question.



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