As part of the in vitro fertilization (IVF) process, you may have embryos preserved through freezing for later use. Here is what you should know about the freezing and thawing of embryos.

How are embryos frozen?

Embryos were traditionally frozen using a ‘slow-freezing’ method where the temperature is gradually reduced using a specially designed machine. However, the survival rates of frozen embryos have skyrocketed since the development of a new freezing technique called vitrification. Vitrifying embryos means that they are frozen at such a rapid rate that there is no time for ice crystals to form inside the cells, which is the main cause of damage, often leading to embryo death.


Vitrification

Instead, the water inside the cells freezes at -20,000°C per minute into a glass-like state. Because no ice is formed, technically vitrification isn’t freezing; it is referred to as ‘cooling’. This super-fast rate of cooling is achieved by holding the embryo in extremely small volumes of liquid and plunging it into liquid nitrogen which keeps the embryo perfectly preserved at -196°C.

Before embryos can be frozen, they need to be gradually dehydrated to remove the water in and around the cells. They are moved individually through different liquids which vary in their concentration of cryoprotectant (solutions that replace the water and protect the cells). Although cryoprotectants protect the embryos from ice damage, they can also be toxic. This means that the embryos can only be in contact with the media for very specific amounts of time to ensure the cells are sufficiently preserved, without excessive exposure which can do more harm than good.

The water in the cells is gradually drawn out and the cryoprotectant is slowly transferred in. It is normal for a blastocyst to ‘collapse’ during freezing because it contains a central cavity filled with water which needs to be removed before freezing. When it is ready, the embryo is picked up in a tiny amount of liquid and transferred onto a vitrification device which usually contains a microscopic well or trough that the embryo sits in. The device is quickly plunged into a canister of liquid nitrogen which instantly freezes it in its equilibrated state.

Read: 6 Days in the IVF Lab

How are embryos thawed?

When the embryo is ready to be used for transfer it can be thawed out. Although because the embryo has been vitrified into its glass-like state and there is no ice involved, technically the embryo isn’t thawed; it is ‘warmed’ The process of warming embryos is essentially the reverse of cooling them. They need to be gradually rehydrated by moving them through solutions designed to draw out the toxic cryoprotectants and replace the water in the cells.

Embryo Warming Survival Rates

If you have more than one embryo cryopreserved, they will be thawed in order of their quality with the highest grade embryos being chosen first. (Read more about embryo and blastocyst grading.) Embryos are actually very resilient and they handle the freeze-thaw process very well. The survival rate should be around 95% if the embryo is handled correctly. However, immediately after the embryo has been warmed it is often collapsed and looks squashed so it is difficult to tell whether it is still viable. Many clinics leave the embryo in an incubator for a few hours or even overnight after warming so it can re-expand and be assessed fully to get a better idea of its potential. Other clinics may thaw the embryo immediately before transfer because they believe that the embryo should be given the opportunity to re-expand inside the uterus which might be a more favorable environment for it to recover in.

It is also possible to refreeze an embryo after it has been thawed. For example, this might be done if the parents want the frozen embryo to be genetically tested. In this case, it will be thawed and biopsied then refrozen while we wait for the results.

Read: An Intro to Preimplantation Genetic Screening (PGS)

What can go wrong with embryo freezing?

The most common problem is that the embryo doesn’t survive the freezing/thawing process. This may be due to human error which can mean that the embryo is exposed to toxic cryoprotectants for longer than it should be, or the embryo may not be frozen correctly which causes damaging ice to form inside the cells. It could also be due to damage caused by many accidental mini freeze-thaw cycles during storage. This can happen if lots of embryos are stored in the same place; all the embryos have to be taken out of the freezer (dewar) to find a specific patient 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.

Alternatively, the embryo may not survive simply due to its biological fate or its genetic makeup. The embryo may be low grade and perhaps should not have been frozen in the first place. However, 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.

Sometimes, the embryo can just partially survive. This means that some of the cells look healthy and some less so. An embryo is often considered suitable for transfer if at least half of the cells have survived. Blastocysts are made up of hundreds of cells and when they are thawed they look shrunk and crumpled which makes it difficult to grade accurately. Because of this, the embryo will be left to re-expand in the incubator and the embryologist will assess how well it rehydrates back to its normal size which usually happens in 2-4 hours. Some studies have found that blastocysts which re-expand more quickly have a higher implantation potential than those which have not fully rehydrated after several hours.

Read: 5 Things Your Embryologist Wants You To Know About Your IVF Cycle

What are the benefits of embryo freezing?

With such amazing improvements to embryo freezing techniques and great survival rates, many clinics now opt to do 100% frozen embryo transfers (FET) and there are many reasons why. Most importantly, it gives your endometrium chance to develop to the optimal thickness so it can be in perfect synchrony with the embryo. This aims to mimic the process of a natural conception to give the embryo the best chance of implanting.

Freezing embryos also freezes time. The age you are when you start your IVF cycle is one of the biggest factors which determines success and it rapidly declines with every year your pregnancy is delayed. Freezing your embryos gives you the peace of mind that your success statistics are locked in which means you can take your time with as much breathing space as you need — assisted reproduction is a very stressful process after all.

It is also important in encouraging single embryo transfers. Having multiple embryos transferred significantly increases the chance of twins and even triplets which can be very dangerous for both the mother and the babies. Now that embryos can be frozen with great success, embryos can be transferred one at a time which optimizes the chance of a positive outcome. The aim will always be to produce a single, healthy live birth.

Every patient is unique

Remember that embryo freezing and thawing are very intricate processes and the complexities cannot be fully explored here. There are advantages and limitations in all situations which need to be carefully considered as every patient and every embryo is unique. Trust your embryologist but don’t be afraid to ask for more information if you feel you would benefit from it—they are your embryos.