Egg freezing is a relatively new technology; it has previously been difficult to preserve eggs because they are a lot more delicate to freeze than embryos. Vitrification is a new freezing method that has allowed advances in egg cryopreservation, although some clinics have had success using the slow freezing method.

Why Egg Freezing?

The primary reason for egg freezing is fertility preservation, when a woman is not ready to have children but feels her biological clock ticking, or if a woman has a serious illness and treatment will leave her infertile. This is an option for some women with cancer who are about to undergo chemotherapy, although there may not be enough time to do a fully stimulated cycle before chemotherapy treatment begins. Fertile Hope is an organization that helps women with cancer by funding part of the in vitro fertilization (IVF) cycle.

Eggs can also be frozen in the event that no sperm is available on the day of egg retrieval or if a couple has moral objections to freezing embryos.

What is Egg Vitrification?

Egg vitrification is similar to embryo vitrification and involves dehydrating the eggs in a series of cryoprotectants and then plunging them into liquid nitrogen in a very small volume. Vitrification literally means “turning to glass.”

A stimulated cycle is usually required before egg freezing, with the exception of some women who are about to undergo treatment for cancer that cannot be delayed. This involves four to six weeks of medication to stimulate the ovaries to produce many eggs at one time. These eggs are surgically removed once the follicles have reached full maturity.

The mature eggs are processed through solutions containing high concentrations of cryoprotectants. They are then quickly placed in a tiny drop of fluid barely larger than the egg itself and placed onto a “leaf” that looks like a small spatula. The leaf is plunged into liquid nitrogen before being placed into a secure holder labeled with the patient’s identifying information.

When embryo transfer is desired, the eggs are thawed and fertilized using intracytoplasmic sperm injection (ICSI) before being cultured in the lab for several days and transferred into the uterus as embryos.

How Does Vitrification of Eggs Work?

Freezing biological material can be challenging because of the formation of ice crystals. Water expands when frozen, causing damage to the surrounding tissue if it’s not fully removed before freezing. Any type of cryopreservation involves dehydration of the tissue, and vitrification uses high concentrations of these dehydrating solutions. During the freezing process, all the water inside the cell is replaced with the cryoprotectant solution, enabling the cells to be plunged into nitrogen with no ice crystal formation.

Is Egg Vitrification Safe?

As of November 2016, there have been several thousand babies born from frozen eggs. These babies appear to be perfectly healthy and normal. When scientists begin using a new technique, we never know the long-term effects. However, historical use of egg/embryo freezing leaves us confident that this method is entirely safe.

Why is Vitrification a Better Option for Egg Freezing?

Because vitrification uses higher concentrations of cryoprotectants, it’s more efficient at removing the water from the egg cell. This, coupled with the faster freezing rate, allows the eggs to be further protected from the damaging effects of the freezing process. The old way of freezing was slower and gentler but often did not fully dehydrate the cells and left the eggs and embryos susceptible to damage.

What is the Success Rate from Thawed Egg IVF?

The success rate from using frozen eggs depends on a variety of factors, most important, the age of the woman at the time of freezing. Other factors include how many eggs are available and underlying fertility problems that could inhibit pregnancy. It’s usually recommended that at least 10 eggs be stored to give the best chance of a viable pregnancy.

In general, if the eggs survive the freeze/thaw process and fertilize normally, they are just as likely to grow as embryos created from fresh eggs are. The pregnancy rates are slightly lower overall, but egg freezing offers good prospects for people with normal fertility at the time of freezing.

How Will Vitrification Help Success Rates in Donor Egg Programs?

It’s possible that, in the future, we will see a wave of egg banks that offer frozen eggs at a lower price than doing a fresh IVF cycle. Egg vitrification is not necessarily going to help success rates in the future because embryos from fresh eggs may be more likely to implant than frozen eggs are. However, egg vitrification may provide a more affordable and accessible option for some people. One positive aspect of using frozen eggs is that they are readily available and screened, so their use alleviates a lot of the waiting time and synchronization between the donor and recipient.

Can Thawed Eggs be Refrozen as Embryos?

Yes. Once we thaw the eggs and fertilize them, we usually see a pattern of growth very similar to fresh eggs/embryos. When we get to Day 5 of culture and transfer the best blastocysts, we freeze the spare embryos that are of good quality. The freeze/thaw success rate of these embryos is similar to embryos that come from eggs that have never been frozen.