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Cryopreservation (embryo, egg and sperm freezing)

Embryo freezing

Treatments such as IVF, ICSI and Egg donation often generate embryos that are surplus to the immediate requirements of the treatment. If these 'spare' embryos are of sufficiently good quality they can be frozen and placed in storage for your future use.

Why are surplus embryos created? IVF and related treatments normally involve stimulation of the ovaries to produce a number of eggs. We aim to fertilise all of the eggs removed in order to have a selection of embryos to grow, as not all embryos have the potential to develop. As they are growing we are able to select the best quality for replacement. In many cycles more embryos than the one or two necessary for replacement are of good quality, and have the potential to survive freezing and be viable once they are thawed. If we place these embryos in storage this provides another possibility of pregnancy without having to repeat the process of ovarian stimulation.

Do we have to freeze spare embryos? No, it is not necessary to freeze but we normally recommend this as it provides the opportunity of a pregnancy if the fresh replacement doesn't work - without having to go through the whole treatment again.

Can all embryos be frozen and stored? Unfortunately not every patient will have embryos suitable for freezing. Any spare embryos will be cultured for 6 days after egg collection to see if they develop to the blastocyst stage; this is referred to as ‘blastocyst culture’. This enables us to select the best quality embryos, as blastocysts have a greater potential to develop and implant. Only the embryos that make good quality blastocysts can be frozen, as we know from experience that only these embryos are capable of surviving the freeze/ thawing process.

How are the embryos frozen and is there any danger to them? At the CRM we use a process known as ‘vitrification’ to freeze embryos. This is a highly rapid freezing process which helps achieve higher embryo survival rates, as it reduces the formation of damaging ice crystals, inherent in other freezing methods.

The selected embryos are placed in a ‘cryoprotectant’ and are inserted into a ‘straw’ that is labelled with your unique details. At the embryo of the freezing process, the straws are placed into large storage tanks filled with liquid nitrogen at -196oC. Embryos can be frozen at various stages of their development e.g. day 1 (pronuclear stage), day 2/3 (4-8 cell stage) and day 5/6 (blastocyst stage). However, at the CRM Coventry we aim to freeze embryos at the blastocyst stage as we believe this results in better embryo survival rates.

About 85% of embryos survive this process and there is no evidence to date that the freezing process is harmful to the ability of the embryo to develop into a normal baby. Replacement of embryos after thawing from the frozen state has been carried out since 1983. However, as with all assisted conception treatments it is important that we continue to collect information on the outcome of such treatments.

How long can embryos be stored for? Ten years in the first instance. You can extend the storage period for longer providing you have renewed your consent to enable us to do so. There is no known deterioration in the health of the embryo with time.

Will the embryos survive the thawing process? As has already been stated approximately 85% of the embryos will survive, and we have no way of knowing until they are thawed if they are going to do so.

How long does it take to thaw the embryo? Only a few minutes. Normally, the embryos are taken out of storage only a short while before they are going to be replaced. Sometimes we grow the embryos for a day or so after thawing to see if they can develop further. We will do this particularly if the embryos were less than four cells stage when they were frozen.

How many embryos will we thaw? Having a multiple birth is the single greatest health risk associated with fertility treatment. Multiple births carry risks to both the health of the mother and to the health of the unborn babies. Twins or triplets are more likely to be premature and to have below-normal birth weight. For this reason, in the majority of cases we will thaw a single embryo initially and assess its viability (i.e. the number of cells which have survived from the original embryo). We will thaw additional embryos only if the first embryo does not survive the thaw. Some patients may be eligible to have a double thaw and transfer in their FET cycle. In all cases, the number of embryos to be thawed will be discussed with you at the time of planning your treatment.

How likely is the replacement of frozen embryos to result in a pregnancy? Our current cumulative FET results are showing a clinical pregnancy rate per embryo transfer of 32%. Similar results are seen in both our natural and HRT stimulated FET cycles. For further information, please see our results section.

What are your obligations? The storage of embryos is governed by the HFEA and we are obliged to comply with the law as it relates to this storage. This means that we cannot extend the storage of embryos without your written consent. It is vitally important that the Embryology team are informed of any change in contact details. If you do not ensure the CRM are up to date with all contact information and we are unable to contact you, this may result in your embryos being removed from storage.

It is also important that you are aware that the payment of storage fees will not guarantee that your embryos will remain in storage if you do not have the appropriate consent for embryo storage in place.

In addition, non-payment of storage fees may result in your embryos being removed from storage, even if the consent to store embryos has not expired.

What if I separate from my partner? Embryos are created from a sperm and an egg. Therefore the provider of the sperm and the egg each have to individually consent to the terms under which the embryo is stored. Both the female and male partner (unless donor eggs or sperm is being used) will be required to complete a special consent form to indicate their wishes for the use of stored material. Embryos can only be used if both male and female partners consent to the use which is being proposed. It is also important to consent to what you want to do with the embryos in the event one or other of you dies.

What process is involved in the replacement of the embryos? Embryos can be replaced in either a natural cycle (no drugs involved) or in a cycle in which the lining of the womb is stimulated with hormonal therapy (if you are going to undertake this process, details of the drug regime will be provided to you).

Egg Freezing

Egg freezing consists of freezing of viable eggs (oocytes) prior to fertilisation. Stored eggs are then thawed and fertilised with sperm for the purpose of treatment at some date in future.

This process may be considered for a woman who is about to undergo certain medical treatments, such as for cancer, which affects the ovaries thereby preventing her from producing eggs in the future.

To obtain eggs, ovarian stimulation and egg collection is performed as for standard IVF (see IVF info). Eggs are then frozen within a few hours of their collection. Eggs that have been frozen and are used for treatment after thawing are injected as in ICSI to achieve fertilisation. The timing and method of any subsequent embryo transfer into the uterine cavity are as for frozen-thawed embryo transfer.

There are still much lower numbers of egg freeze cycles performed in the UK in comparison with frozen embryo cycles and egg freezing is not routinely performed at the CRM Coventry. However, techniques for freezing and thawing eggs for treatment are improving since the introduction of vitrification (rapid freeze) of eggs and embryos, which we are currently using at the CRM.

For women who are having cancer treatment

This is a very difficult time for you. There is a fair chance that the chemotherapy will not make you infertile. If you are about to embark on cancer treatment that may make you sterile and, particularly if you have no children, then you should consider your options carefully. It is possible to either freeze your eggs, or to freeze embryos (created with your partner’s sperm). If you have a partner then creating embryos and freezing them gives a better chance of a pregnancy than freezing eggs. You must bear in mind that egg harvesting for either freeze type will delay your cancer treatment and unfortunately still gives no guarantees of being able to have a child in the future. We are very happy to see you to discuss these issues whatever your decision.


Sperm Freezing

Sperm can be stored for a number of reasons, including:

  • To store donated sperm (sperm banking)
  • For use by couples due to undergo infertility treatment if the man finds it difficult to ejaculate on demand, which may result in their inability to produce a sample on the day of egg collection (back-up sample)
  • To provide storage after sperm have been surgically removed from the epididymis (PESA) or testes (TESA)
  • Before a vasectomy
  • Prior to cancer treatment that may compromise fertility

Sperm is stored in a similar way to that referred to above for embryo freezing. The initial storage period for sperm is normally 1 year if storage is for back-up for your treatment. In cases such as pre-chemotherapy storage the standard storage period is 10 years. The period can be exceeded in certain circumstances, up to a maximum of 55 years. Your clinician will be able to explain whether you can do this, and how long you may be able to store your sperm.

Tests and consent prior to storage of eggs, embryos or sperm

The following tests are required for anyone wishing to store:

  • HIV
  • Hepatitis B & C
  • Cytomegalovirus (CMV)

Because of the regulatory requirements special consent forms have to be completed ahead of the storage. These indicate how long you wish to keep the material in storage for, what you wish the material to be used for and what you wish done with the material in the event of your death or if you become incapacitated.

For further information, please see the Andrology section.