Embryo freezing: how it works, storage and what to ask

If you have embryos suitable for freezing, the lab cools them so fast that ice crystals are unlikely to form. They are then stored for possible future use until you are ready for a transfer. Before anything is frozen, your clinic will ask you to sign forms about fees and about what should happen to embryos you do not use.

Your clinic's own plan and forms always come first.

What gets frozen?

After retrieval, eggs are fertilized and grow into embryos in the lab. Each embryo is then transferred, frozen or discarded, based on the plan your doctor made with you. Freezing can give you another chance at pregnancy later, or give time for genetic test results to come back.

Embryos can be frozen at different stages. Some are frozen at the cleavage stage, around day 3. Others are frozen as blastocysts, on day 5 or 6. One research review, drawing on an earlier study, reports that the stage at freezing did not change how well embryos survived. That is a research finding, not a promise for every embryo or every lab.

Embryo freezing is part of most IVF programs. Some people freeze the embryos left after a fresh transfer. Others freeze every embryo and skip a fresh transfer, which is called a freeze-all cycle.

What is vitrification?

Embryo cryopreservation is the medical term for freezing embryos. The newer method is called vitrification. Think of it as flash-freezing.

Here is how it works, in plain words:

• The embryo is placed in a special fluid that protects its cells.
• It is then cooled extremely fast.
• Cooling that fast lowers the risk of ice crystals, so the embryo turns solid without turning to ice.

Ice crystals can harm cells, and an older method, slow freezing, had more trouble with them. Vitrification works much better than slow freezing for both day 3 embryos and blastocysts.

Later, the embryo is warmed (thawed) for transfer. Not every embryo survives freezing and thawing, so ask your clinic how embryos like yours have done after warming in its lab.

Lab updates about freezing

You will hear about your embryos over several days after retrieval. Learn the terms in embryo development and lab reports.

Your freezing update may tell you:

• How many embryos were frozen, and on which day.
• Their grades, if your lab shares them.
• Whether any were biopsied for PGT-A before freezing. A biopsy takes a few cells from the embryo for a genetic test.
• Whether those test results are back or still pending.

A biopsy is not the same as a result. Many clinics biopsy embryos on day 5 or 6, freeze them, and get the results later. Write these details down, or ask for a copy, because you will need them when you plan a transfer.

How long can frozen embryos be stored?

Embryos can be stored for years, and a frozen embryo transfer can take place years after the eggs were retrieved. One research review found that how long vitrified blastocysts stayed frozen did not seem to affect pregnancy or newborn results.

How long your clinic will store them, and on what terms, is set out in your forms.

What storage does, and what it cannot do

Storage keeps your embryos frozen so they are available for possible future use. It does not guarantee that each embryo will survive warming, or that a transfer will lead to a pregnancy.

Some jobs stay with you while embryos are stored. You keep paying storage fees, keep your phone, email and address up to date with the clinic, keep a copy of your signed forms, and look at your written wishes again over time.

Storage paperwork and fees

The forms put your wishes in writing, so the clinic knows what to do if your life changes. Clinics are advised to get these written wishes before embryos are frozen. The forms may ask about hard "what if" moments, such as:

• The death of one or both partners.
• Divorce or separation.
• Not paying storage fees.
• Partners who can't agree later on.
• Losing touch with the clinic for a long time.

You can ask to update your instructions by giving the clinic new written ones, and your clinic should tell you how to do this. Whether a change takes effect depends on what your consent forms require, any agreements you signed, and the law where you live. ASRM, a U.S. professional group, notes that U.S. law does not give clear guidance on some questions about stored embryos, such as when unclaimed embryos may be discarded. It helps to look at your choices again over time, especially once you feel your family is complete.

If you decide to stop storing embryos, choices may include:

• Donating them to another person or couple.
• Donating them to research.
• Donating them for clinical training.
• Discarding them.

What you can choose depends on your clinic's rules and the laws where you live. This page is not legal advice. Ask your clinic, or a lawyer, if anything in the forms is unclear.

Questions to ask about storage

• How much does freezing cost, and how much is storage each year?
• Who holds the storage contract: the clinic or a separate storage company?
• How are fees billed, how does renewal work, and what happens if a payment is missed?
• Where are my embryos stored? Can they be moved to another clinic or storage site, and what does transport cost?
• What happens to my embryos if the clinic closes or changes owners?
• Who do I contact in an emergency, and how will you contact me if something goes wrong with storage?
• How do I update my contact details?
• How do I change my written wishes later, and does my partner need to sign too?
• What is your policy on embryos when the clinic can't reach a patient?

A clinic is not expected to store embryos forever if it can't reach the patient and fees stop. That is why it matters to keep your contact details up to date.

Choosing your next transfer

When you are ready, your frozen embryos can be used in a frozen embryo transfer (FET). Your uterus is prepared first, and an embryo is thawed and placed in it. Read how it works in frozen embryo transfer.

Your doctor may prepare your uterus with hormones, or time the transfer to your own natural cycle. See natural FET and medicated FET.

You and your doctor will also decide how many embryos to transfer. U.S. guidance from ASRM depends on age, how likely treatment is to work for you (your prognosis) and the embryo's stage. For a frozen transfer, it uses your age when the embryos were frozen. With donor eggs, it uses the donor's age. In short:

• An embryo with a confirmed normal PGT-A result (called euploid): transfer one, at any age.
• Under 35 with a favorable outlook: one embryo is strongly encouraged, whatever its stage.
• 35 to 37 with a favorable outlook: one embryo should be strongly considered.
• At older ages, or with a less favorable outlook, the guidance allows more, based on your situation.

A biopsy alone, or a result that is pending, unclear or not normal, is not the same as a confirmed normal result. Transferring more than one embryo raises the chance of twins or more, which carries more risk.

To compare a fresh transfer with freezing all embryos, see fresh vs frozen transfer.

Key takeaway

Before freezing, ask what freezing and storage cost, who holds the contract, and how fees are billed. Read your storage forms closely and keep a copy. Ask how to update your written wishes, and what your forms and local law require for a change.