Freezing embryos during IVF – what is embryo freezing?
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Professor Robert Winston
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Fertility expert and one of the world's pioneers of IVF and Fertility Medicine. BAFTA award-winning television presenter and Member of The House of Lords in the UK.
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Conception / IVF

Freezing embryos during IVF – what is embryo freezing?

An average IVF cycle collects about nine or ten eggs, and approximately 60 percent of these will be fertilised if the sperm are healthy. However, the number of embryos that can be transplanted is limited because of the risks associated with multiple births. So egg freezing for storage and later use (known as cryopreservation) of the ‘spare’ embryos has gained wide acceptance.
In Short
Freezing embryos is appropriate for women who are:

Over 40 years old who cannot afford to wait for another IVF attempt if the first one fails; spare embryos can then be thawed and transferred at monthly intervals.

Facing cancer treatment that may affect the eggs in the ovaries.

Ill during IVF.

Receiving a donor embryo.

Frozen embryos as part of ongoing IVF

In the UK, the HFEA (Human Fertilisation and Embryology Authority) has set an initial time limit of 10 years to store embryos. This means that a patient whose first cycle of IVF is unsuccessful can return for another attempt using one or more of her stored embryos, or a woman who has a child can come back for a second cycle of treatment to attempt another pregnancy with a thawed sibling embryo.

Cryopreservation can also be used to preserve the fertilised eggs of women with malignant diseases such as cancer or leukaemia, who have to undergo radiatiotherapy or chemotherapy that may make them sterile. With the approval of the genetic parents, frozen embryos can also be donated to other infertile couples, or for research.

How are frozen embryos stored?

At very low temperatures, many degrees below that in a domestic deep freeze, the degradation and decay of all biological tissues is slowed right down so a live embryo can be held in suspended animation for many years. Embryos are stored in liquid nitrogen at around -200º C. About 80 percent of the air we breathe is made up of gaseous nitrogen, a relatively inert and safe gas at room temperature. When cooled to around -196º C, it becomes liquid. Plant and animal cells contain water, so an embryo cannot be plunged straight into liquid nitrogen as this would cause jagged ice crystals to form. So the embryos are bathed in a solution of ‘antifreeze’, such as glycerol, called a cryoprotectant, to get rid of most of the water in the cells, so minimising the risk of ice formation.

Embryos are stored in specially designed freezing machines, programmed by computers to freeze embryos (and other tissue) at a precise and usually slow, controlled rate. Once calibrated, the machine reproduces the precise rate of freezing and thawing required, ensuring maximum viability of the tissue to be cryopreserved. Eggs and sperm, and ovarian and testicular tissue each require a different recipe to assure the best storage without damage.

Are there any safety concerns for embryos that are frozen?

Safety is a concern. Embryos seem less likely to produce a pregnancy after freezing and thawing. Clinics have varying data, because there is no standard way of reporting the results of embryo freezing, but human embryos after thawing and transfer may often be only half as likely to produce a baby as those that have been transferred fresh. Certainly, after slow freezing microscopic damage can occasionally be seen. Most embryos are frozen at four- to eight-cell stage. Microscopic inspection after the thawing of what was an eight-cell embryo before freezing frequently shows that some of its cells are dying, missing or fragmented. However, several thousand normal babies have been born after embryo freezing in spite of this, and there is no evidence of any increase in fetal abnormality. But we do not know with absolute certainty what problems may become apparent as these children grow up.

Use of vitrification to protect frozen embryos

One concern is the cryoprotectants in which the embryo is bathed before cryopreservation. There are various different compounds; one of the most common is sucrose, and another dimethylsulphoxide (DMSO) a powerful solvent that penetrates cell walls very quickly, which in high doses has been associated with mutation of the DNA in cells. DMSO could just possibly dissolve other mutagens within it that go on to cause mutation.

To get round some of this problem, there has been recent emphasis on a process called vitrification. This involves the extremely rapid freezing of cells or an embryo after immersion in a concentrated solution of cryoprotectant. As with all procedures this assumes that cryoprotectants are not biologically harmful. Vitrification gives better clinical results for freezing many cells, including sperm, eggs and possibly embryos during early development. While this seems true in the short-term and more embryos survive after vitrification, the long-term effects may be different. Although vitrification is widely regarded as the best and safest process, its potential effect on gene function is poorly understood. One of the problems is that if freezing did occasionally cause mutation, the effect would be unlikely to show up during infancy, such problems usually manifest themselves much later in life. This is not saying that embryo freezing causes cancer or infertility, just that it should still be considered more cautiously than it sometimes is at least until the children produced from frozen embryos are fully grown.

Justifications for embryo freezing

Limitations on embryo freezing are a ‘counsel of perfection’. The pressure on clinics and their patients to do everything to avoid multiple pregnancy is now so great that, after a successful IVF cycle yielding several embryos, more women are having single embryos transferred whilst the rest are stored frozen for a later treatment.

In the absence of clear evidence that embryo freezing is harmful, the following are clearly situations when freezing is completely justified:

  • Women over 40 years old who cannot afford to wait for another IVF attempt if the first one fails; spare embryos can then be thawed and transferred at monthly intervals.
  • Women needing cancer treatment that may affect the eggs in the ovaries.
  • Women who become ill during IVF – for example if they have been a bad reaction to drugs like FSH. When there is serious hyperstimulation it makes sense to delay embryo transfer so that there will not be further deterioration due to the hormones produced by a pregnancy. Embryos can be transferred when the woman has fully recovered.
  • Women receiving a donor embryo, where a period of quarantine is necessary before the transfer of the embryo to ensure that the donor does not have a serious disease like AIDS.
Note
In 2005, a baby in the USA was born from an embryo frozen 13 years earlier.

The Genesis Research Trust

Despite countless breakthroughs in medical science, we still do not understand why some pregnancies will end in tragedy. For most of us, having a child of our own is the most fulfilling experience of our lives. All of us can imagine the desperation and sadness of parents who lose a baby, and the life-shattering impact that a disabled or seriously ill child has on a family.

Professor Robert Winston’s Genesis Research Trust raises money for the largest UK-based collection of scientists and clinicians who are researching the causes and cures for conditions that affect the health of women and babies.

Essential Parent is proud to support their wonderful work. You can learn more about them here.

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DISCLAIMER
This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Essential Parent has used all reasonable care in compiling the information from leading experts and institutions but makes no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details click here.