Vitrification for Egg, Sperm, and Embryo Freezing

cryogenic (frozen) storage for eggs frozen via vitrification
Vitrification can be used to freeze eggs, sperm, and embryos. Science Photo Library / Getty Images
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In the fertility world, vitrification is the process used for cryopreservation (freezing) of eggs, embryos, and sperm. Eggs, sperm, and embryos may be frozen in cases where an individual is not ready to become pregnant but wants to ensure that they can in the future. 

Vitrification is sometimes referred to as "fast freezing." In this process, water molecules are removed and replaced with a higher level of preserving solution before being placed in liquid nitrogen. Freezing occurs in milliseconds and can help prevent cell degradation during the thawing, fertilization, and transfer process. 

Vitrification has improved the success of cryopreservation. The old methods involved a slow freeze, while vitrification is extremely quick.

During vitrification, an embryo or egg is cooled at a rate of about 27,000 degrees F per minute!

What Is Vitrification?

Generally speaking, vitrification is a method of transforming something into a glass-like substance. It comes from the Latin root vitreum, which means glass. Vitrification technology is used to transform sand into glass, to give ceramic pots their glossy finished look, and to stabilize nuclear waste for safer disposal.

In fertility medicine, this method of cryopreservation is called vitrification because it freezes water molecules in cells into a glass-like structure rather than ice crystals.

Until recently, the only method for freezing oocytes (or unfertilized eggs) was a slow-freezing method. This worked okay for freezing sperm or embryos. However, for eggs, the slow freeze process had many problems.

Egg cryopreservation requires cooling an egg cell until it reaches -320º Fahrenheit. This freezing temperature halts the biological process inside the cells, allowing it to be safely stored for future use.

In the slow-freezing method, the egg would be very slowly cooled until it reached the desired temperature. However, this method is not perfect and sometimes led to problems.

  • Ice crystals were a major issue. Eggs contain a lot of water, compared to sperm and even embryos. Freezing eggs led to crystal formation. These crystals broke down the egg.
  • To help minimize the number of ice crystals, scientists would remove some of the water. But it's impossible to remove all the water.
  • When the eggs were thawed, they were damaged and frequently unusable.
  • Fertilization and pregnancy rates for these slow-frozen eggs were low.

With vitrification, the freezing process is so fast that ice crystals don’t have a chance to form. The process has made egg freezing a much more viable option for women.

Vitrification is also being used for embryo and sperm cryopreservation. Research is ongoing, but so far, pregnancy rates seem higher with vitrification compared to slow freezing methods, according to a 2014 study.

How Does Vitrification Work?

Using eggs as an example, vitrification requires high concentrations of cryopreservants, or anti-freeze substances. Because these chemicals are potentially toxic to the egg, the technique requires special care.

The oocyte is first placed in a bath with a lower concentration anti-freeze. The solution also contains some sucrose (sugar) to help draw water out of the egg. Next, the egg is placed in a highly concentrated bath of anti-freeze for less than one minute, while being instantaneously frozen.

The eggs can then be stored in special cryogenic freezers, made for this purpose. The eggs are held in tiny straws.

When it's time to thaw the egg, the oocyte must be warmed quickly and removed from the solution immediately.

Once thawed, the egg can be fertilized using in vitro fertilization (IVF) with intracytoplasmic sperm injection (ICSI). ICSI involves taking a single sperm and injecting it directly into the egg. Regular IVF isn’t possible because the freezing process hardens the egg's outer membrane.

When Is Vitrification Used?

Vitrification can be used to cryopreserve embryos, eggs, sperm, and even ovarian tissue. Situations where vitrification may be used include:

Preserve Fertility Before Cancer Treatment

Some cancer treatments cause sterility. If a woman freezes her eggs, or a man freezes his sperm, he may be able to use the egg or sperm after cancer treatments to have a child.

Ovarian tissue freezing is a relatively new technology, one that is especially helpful for young girls. If a girl hasn’t gone through puberty, it’s not possible to retrieve mature eggs from her ovaries. However, ovarian tissue can be frozen. The technique is still experimental.

Medical Conditions

Vitrification may also be used if a woman has a medical condition that may have an impact on future fertility. If a woman is at risk for early menopause or primary ovarian insufficiency (also known as premature ovarian failure), she can freeze her eggs when she’s younger and still has healthy eggs left.

Freeze Embryos After IVF

Any extra embryos left during an IVF cycle can be cryopreserved with vitrification. So far, research has found greater success with embryos frozen via vitrification, as opposed to the slower process. In fact, success rates using frozen embryos compare so favorably even with fresh embryo transfers that experts are debating whether frozen embryos should be used for all IVF procedures.

Egg donor banks

It used to be that if you needed an egg donor for IVF, the donor had to go through the fertility treatment process at the exact same time as you did. It involved regulating both of your cycles to occur at the exact same time.

It is an expensive and complicated process. People still do “fresh donor” cycles. But with egg banks, thanks to vitrification technology, you can get previously frozen eggs to use during IVF. The cost is slightly less.

Extend Childbearing Years

Egg freezing in order to avoid age-related infertility is still controversial. The American Society of Reproductive Medicine is against the idea, saying that the technique is too new. Research hasn’t clearly shown the potential benefits outweigh the risks.

The safety of freezing eggs for years at a time still isn't well understood. The majority of the research focuses on eggs frozen for a matter of hours or months.

From the other side, some fertility doctors marketing vitrification claim that the latest research shows that the technique is successful. They believe it is time to release the new technology to those that want to try it.

How Effective Is Vitrification?

Vitrification appears to have better post-thaw survival rates when compared to slow freezing methods. Previous research has found an 87% survival rate with vitrification compared to a 70% rate for slow freezing.

  • A 2014 review concluded that while vitrification has led to improvements in the efficacy of egg preservation, there are a number of developments that may help further improve egg survival, pregnancy rates, and live birth rates.
  • One 2018 study found that the vitrification and warming process led to decreased pregnancy and live birth rates compared to egg cells that had not gone through the vitrification and warming process. Researchers suggested that vitrification may have negative effects on fertilization rates, embryo development, embryo quality, and live birth rates.
  • According to 2019 research, pregnancy and live birth rates were better with eggs, sperm, and embryos that were frozen with vitrification. Also, there doesn’t seem to be an increased risk of birth defects in the children. However, the technology is new. Much more research needs to be done.

Future improvement in vitrification techniques may include less cytotoxic vitrification solutions and devices to help standardize the vitrification process.

What Are the Risks of Vitrification?

So far, the research looks promising when comparing slow-freeze to vitrification. However, further research is warranted to look at the long-term development of infants born as a result of the vitrification process. Some potential risks of vitrification include:

  • Egg retrieval risks: There are also some risks related to ovarian stimulation and egg retrieval. While the chance of a serious problem is small, the egg retrieval process does include a risk for pain, ovary or pelvic infection, and injury to the bladder, bowel, uterus, or ovaries.
  • Exposure to cryopreservants: There are concerns that overexposure to cryopreservants could be harmful to the eggs, sperm, or embryos. New methods are always being looked at to further reduce the time they are exposed to the potentially toxic chemicals. 
  • IVF-related risks: Women who then undergo in vitro fertilization using vitrified eggs then face the risks associated with the IVF process, which include the risk of a multiple pregnancy.
  • Ovarian hyperstimulation syndrome: Data on the effects of these practices remains somewhat limited, but it may pose a risk for ovarian hyperstimulation syndrome (OHSS). In mild to moderate cases of OHSS, women may experience symptoms such as nausea, headaches, fatigue, irritability, breast tenderness, and abdominal pain. In more severe cases, it may result in vomiting and dehydration, shortness of breath, and blood clots. 

There are also emotional risks associated with any fertility treatment. Vitrification can offer the possibility of a future successful pregnancy, but it is not guaranteed.

A Word From Verywell

Also, it’s important to remember there are no guarantees of pregnancy success. Not every cryopreserved egg or embryo will survive the warming process. Not every thawed egg will become fertilized. Not every embryo will develop and be healthy enough to transfer.

This is extremely important to understand if you’re freezing your eggs to extend your childbearing years. Freezing your eggs may help you become pregnant later, but there is no method or process that offers guaranteed success.

9 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Rachel Gurevich, RN
Rachel Gurevich is a fertility advocate, author, and recipient of The Hope Award for Achievement, from Resolve: The National Infertility Association. She is a professional member of the Association of Health Care Journalists and has been writing about women’s health since 2001. Rachel uses her own experiences with infertility to write compassionate, practical, and supportive articles.