Embryo freezing is a procedure that allows embryos to be preserved for later use. The first successful pregnancy resulting from freezing a woman’s healthy embryos was in the 1980s. Since then, many embryos have been frozen for later use.

The embryos may be stored to enable a future pregnancy, to donate to others, for medical research or for training purposes.

The process begins by using hormones and other medications to stimulate the production of potentially fertile eggs. The eggs are then extracted from the woman’s ovaries to either be fertilized in a lab or frozen for later use.

Successful fertilization may lead to at least one healthy embryo, which can then be transferred to the woman’s womb or uterus. Hopefully, the embryo will develop and the woman can carry the developing infant through pregnancy to a live birth.

Since fertilization often results in more than one embryo, the remaining embryos can be preserved through freezing.

What is an embryo, and how is one created?

According to the Oxford Living Dictionaries, an embryo is “a human offspring during the period from approximately the second to the eighth week after fertilization.”

Before freezing can take place, suitable embryos have to be created. To create an embryo in the laboratory, the eggs must first be harvested and fertilized.

First, the woman will be given hormones to make sure she ovulates correctly. She is then given fertility medications to increase the number of eggs she produces.

In the hospital, a doctor will extract the eggs, using an ultrasound machine to ensure accuracy. The eggs may be frozen or used at once.

If the woman wishes to become pregnant at once, in vitro fertilization (IVF) or intra-cytoplasmic sperm injection (ICSI) may be used to fertilize the egg.

During the process of IVF, the eggs are exposed to sperm and the mixture is cultured in a laboratory. Fertilization may take 16 to 20 hours. The fertilized eggs are called embryos.

An embryologist will monitor the development of the embryos over the next 6 days, after which a suitable embryo may be chosen for implantation.

In ICSI, once the eggs have been extracted, a single sperm is injected directly into an egg. This may be done if there is a problem with the sperm or if past attempts at IVF have been unsuccessful.

While one embryo can be used for pregnancy, others may be frozen.

How is an embryo frozen?

The main goal of embryo freezing is to preserve the embryo for later use. The biggest problem is the water within the cells. When water freezes, crystals can form. This expansion can burst the cell, causing it to die.

To prevent this happening, the water in the embryo’s cells is replaced with a protective substance called a cryoprotectant. The embryos are left to incubate in increasing levels of cryoprotectant before they are frozen.

Once most of the water has been removed, the embryo is cooled to its preservation state through one of two methods of embryo freezing:

  • Slow freezing: This involves protecting the embryos from damage in sealed tubes and then slowly lowering the temperature in the tubes. This prevents the embryo cells from aging and becoming damaged. Embryos can last much longer in their frozen state than in their fresh state. However, slow freezing is time-consuming, and it requires expensive machinery.
  • Vitrification: In this process, the cryoprotected embryos are frozen so quickly that the water molecules in the embryos do not have time to form ice crystals. This helps to protect the embryos and to increase their survival rate during thawing.

After freezing, the embryos are stored in liquid nitrogen until they are needed for future use.

Success rates of thawing frozen embryos

The process of thawing an embryo is relatively successful. Some research has indicated that embryos frozen through vitrification have a better chance of survival, both at the freezing stage and during thawing.

Side effects of embryo freezing

Any risks and side effects involved in embryo freezing usually happen during the process of extracting the eggs from the woman’s body.

Common side effects from extracting embryos for freezing are typically mild and temporary.

They include:

  • Cramping or bloating
  • Feelings of fullness
  • Bleeding
  • Changes in vaginal discharge
  • Infection
  • Overstimulation of the ovaries

How long can embryos be frozen for?

In theory, a correctly frozen embryo can remain viable indefinitely. The embryos are held in sealed containers at temperatures of -321ºF. At this temperature, almost no biological processes such as aging can occur.

There are examples of successful pregnancies that have resulted from eggs that had been stored for up to 10 years. There is no long-term research into embryo freezing, because this procedure has only been carried out since 1983.

Some countries choose to regulate the length of time an embryo can be stored. Freezing and storing embryos is also expensive, and each clinic has its own rules about what happens if a woman can no longer use her own embryos or keep them frozen.

Frozen or fresh embryos?

A study posted in the International Journal of Reproductive Medicine looked at the results of over 1,000 cases of embryo transfer using either fresh or frozen embryos.

The results found no statistical difference between using fresh and frozen embryos for transfer. The study noted that frozen embryos could also be used for additional embryo transfers in the future while fresh embryos could not.

Other research suggests that frozen embryo transfer may be better than with fresh embryos. A recent study compared fresh and frozen embryo transfer. The results indicate that frozen embryo transfer is associated with a higher rate of pregnancy, and better outcomes for both the mother and the embryo.

Who is embryo freezing best for?

Embryo freezing can be used by any woman, but there are certain groups who may find it more beneficial than others.

These include women with genetic disorders of ovarian sensitivities, those who are due to undergo chemotherapy, and those who take medications that affect fertility.

Women who are approaching advanced reproductive age and are not ready for children yet may also benefit from freezing embryos for later use.

“Freeze-all” cycles

In a “freeze-all” cycle, the embryo is extracted from a woman’s body to be frozen for storage.

This procedure may be offered to women who have a higher risk of ovarian stimulation syndrome. This rare and potentially dangerous condition happens when a woman is receiving stimulating hormones to increase egg production.

There are ways to reduce the chance of developing this condition. For example, doctors may freeze the embryos and transfer them when the woman’s ovaries are no longer stimulated.

Women who are going to receive a medical treatment that may affect their ovaries, such as chemotherapy, may choose to store their embryos.

Sometimes, a “freeze-all” cycle is used to test an embryo for a genetic disorder. This involves removing a few strands of DNA from the embryo and testing their chromosomes. Embryos with a standard set of chromosomes are more likely to have a successful implant. Pre-screening can ensure that future offspring have a lower chance of developing genetic diseases.

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