Embryos are so delicate and the freezing/thawing process is so fiddly that there can be a whole host of things that go wrong. Luckily, these problems are rare but it is important to know what to expect and to be aware of important questions you may want to ask your embryologist.
The Embryo Doesn't Survive
The most common problem is that the embryo doesn’t survive the freezing/thawing process. This may be due to human error, the quality of the embryo, or simple biological variation.
By human error we mean poor technique from the scientist. The freezing and thawing process must be done under tightly controlled conditions, with high quality equipment, careful embryo handling and strict timings. Any problems with any of these factors can impact the embryos survival. For example, the embryo must be submerged into a protective fluid which stops damaging ice crystals forming before freezing, but if it is exposed to this solution for too long it can be toxic and kill the cells. Any sort of distraction or lapse in concentration can be a death sentence for embryos. Subsequently, it takes a long time to train to freeze embryos and it is usually only performed by more senior embryologists.
There are also more obvious dangers to the embryo during freezing and thawing because it is so small and delicate. It could be damaged by being mishandled, or it could be loaded onto the freezing device incorrectly which means it does not reappear during thawing – having most likely floated away out of sight.
Alternatively, the embryo may not survive simply due to its biological fate or its genetic makeup. Most clinics have very strict criteria to determine which embryos should be frozen and which shouldn’t. If the embryo is low grade it is much more likely to die during freezing because it is just not strong enough to withstand the stress (and even less likely to go on to produce a pregnancy), so it is recommended that these embryos are discarded. Freezing low quality embryos is rarely advised because it means that a patient pays for a frozen embryo transfer (FET) cycle and takes hormonal stimulation to prepare for the transfer, only to be disappointed because the poor quality embryo does not survive after being thawed. There are however, some people who request that all embryos with even the smallest chance of making a baby should be frozen… just in case. Remember though - even high grade embryos can die after freezing for no known reason and through nobody’s fault. This is one of the most frustrating aspects of working under biology’s control.
Problems during storage
Embryos can also be damaged during storage. The embryos must stay frozen at precisely -196°C to keep the cells perfectly preserved. Any fluctuations in temperature can results in accidental mini freeze-thaw cycles where the embryo thaws out just a little and then refreezes again. This can happen if lots of embryos are stored in the same place because all the embryos have to be taken out of the freezer (dewar) to find a specific one and it causes the temperature of all of those embryos to increase ever so slightly. Although the change in temperature is only brief, the embryo is frozen in such a tiny volume of liquid that it can have a substantial effect on the embryo’s survival.
The Embryo Partially Survives
Sometimes, the embryo can just partially survive. This means that some of the cells look healthy and some less so. This is of course less than ideal but these embryos can still go on to produce a healthy pregnancy. It is normal for an embryo to look a little bit ‘tatty’ after it has been thawed because the cells have been through a lot. However, the difference between a healthy embryo and a degenerating embryo is clear. Keep an eye out for cells that are dying which you can see becoming very dark and grainy.
An embryo is often considered suitable for transfer if at least half of the cells have survived. After the embryo has been thawed, the embryologist will inspect it under the microscope to determine how healthy it looks. Blastocysts (day 5 embryos) are made up of hundreds of cells and when they are thawed they look dehydrated and crumpled which makes it difficult to grade accurately. Because of this, the embryo will be left to rehydrate in the incubator and the embryologist will assess how well it re-expands back to its normal size, which usually happens in 2-4 hours. Some studies have found that blastocysts which rehydrate more quickly have a higher implantation potential than those which have not fully rehydrated after several hours… something which may be useful to ask your embryologist about if you are having a frozen embryo transfer.