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Centre for Reproductive Medicine

About Us

Treatments we offer
What to expect
Ovulation Induction (OI)
Intra-Uterine Insemination (IUI)
Egg Share Programme
Surgical Sperm Retrieval (SSR)
Cryopreservation (embryo and sperm freezing)
Frozen Embryo Replacement
Egg Donation
Pre conception advice
Tubal surgery

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Egg Donation

Embryo and Sperm Storage


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Cryopreservation (embryo, egg and sperm freezing)

Please also see our patient information booklet for further information

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 most cycles more embryos than the one or two necessary for replacement are of good quality and have the potential to survive freezing and storage and be viable once they are thawed. If we place these embryos in storage this provides another possibility of pregnancy without having to go through the process of ovarian stimulation.

Do we have to freeze spare embryos? No, it is not necessary to freeze but we normally recommend this at 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? Only good quality embryos will survive the freezing and thaw process. If you have spare embryos which are not good quality, the processing of these embryos will be discussed. If you feel strongly that you would like them placed in storage even though their potential to survive the freeze / thaw process is very poor we will happily discuss this with you.

How are the embryos frozen and is there any danger to them? The embryos are selected, placed in 'cryoprotectant' and inserted into a 'straw' that is labelled with your unique details (this part of the process is witnessed to avoid any error). The straw is then processed so that it reaches the storage temperature of -196 degrees. The straws are then placed into a large storage tank filled with liquid nitrogen. Embryos can be frozen at various stages in their development e.g. day 1 (pronuclear stage), day 2/3 (4-8 cell stage) and day 5/6 (blastocyst stage).

About 70% of embryos survive this process and there is no evidence 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 1986. It is not known how many babies worldwide have been created in this way but probably many thousands of babies have been born using this technique. As far as we know there is no known increase in abnormalities resulting from this treatment. However, as with all assisted conception treatments it is important 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 70% 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? Normally, we thaw two initially and assess their viability i.e. the number of cells which have survived from the original embryo. If either one or none of the first two embryos are viable we will thaw the next one or two embryos until we have two viable embryos available for replacement into the womb. This will be discussed with you at the time os planning the treatment.

Extended culture after thawing (Blastocyst replacement) Increasingly we are thawing a group of embryos and culturing them for several days until they reach the Blastocyst stage. This has the advantage that we are better able to select the embryo that has, theoretically, the greatest potential for further development and implantation. The disadvantage of this technique is that fewer embryos may survive if placed in culture for several days. If we begin this process with too few embryos it is possible than none will develop further or they will deteriorate in culture, and there may be no embryos to replace into the womb. To undertake this process and give it the best chance of working we normally require five or more embryos.

How likely is the replacement of frozen embryos to result in a pregnancy? Our live birth rates for replacement of frozen embryos have fairly consistent over the years at 10-12% per thaw cycle. This is less than some units because we tend to freeze a larger proportion of spare embryos in order to give more chances of a pregnancy and our policy for thawing has been to replace the first two embryos which have an indication of viability. As stated above, we are increasingly culturing the thawed embryos for several days and replacing them at the blastocyst stage. Although this means that there are likely to be fewer surviving embryos, those that do are more likely to result in a pregnancy and our early results indicate almost a doubling of the pregnancy rate using this technique.

What are your obligatoions? 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 beyond ten years without your written consent. Therefore it is most important that you remember if you have frozen embryos in storage because if you change address or your personal circumstances change we may not be able to track you down unless you have informed us. If you have not signed a consent to extend the storage then the embryos will have to be taken out of storage and allowed to perish if they have passed their year storage limit.

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 24 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.

Egg freezing should still be regarded as experimental. There are still much lower numbers of egg freeze cycles performed in the UK in comparison with frozen embryo cycles. Techniques for freezing and thawing eggs for treatment are improving since we have started the process of vitrification (rapid freeze) of egg and embryos.

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 sterile. If you have a partner then creating embryos and freezing them gives a better chance of a pregnancy than freezing eggs but will delay your treatment. If you are about to embark on cancer treatment that may make you sterile and, particularly if you have no children, the you should consider your options carefully, bearing in mind that egg harvesting will delay your cancer treatment and gives no guarantee of a child in the furture.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:

Sperm is stored in a similar way to that referred to above for embryo freezing. The inital 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 clinican 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:

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.

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