Embryo Freezing (Cryopreservation): How It Works, Risks, and Success
Updated: 6 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026 · How we review
Quick Answer: What is embryo freezing?
Embryo freezing, or embryo cryopreservation, is a process that freezes fertilized eggs and stores them in liquid nitrogen at −196 °C for later use. It is most often done after IVF or ICSI to save extra embryos, or to preserve fertility before treatments like chemotherapy. Frozen embryos are later thawed and transferred into the uterus. [1]
What the research says
Freezing all embryos gives about the same chance of a baby as a fresh transfer. A review of 8 randomized trials with 4,712 women found little or no difference in cumulative live birth rate (odds ratio 1.08); ovarian hyperstimulation syndrome fell from about 3% to 1% with the freeze-all approach. (Zaat et al., Cochrane Database of Systematic Reviews, 2021) [2]
Vitrification protects embryos better than slow freezing. Pooled data on 8,824 embryos showed significantly higher survival after thawing with vitrification, for both day-3 embryos (odds ratio 15.57) and blastocysts (odds ratio 2.20). (Loutradi et al., Fertility and Sterility, 2008) [3]
How the uterus is prepared for a frozen transfer may affect pregnancy risks. Transfers in hormone-programmed cycles were linked to higher odds of preeclampsia (odds ratio 2.11, 8 studies) and placenta accreta (odds ratio 6.29, 2 studies) than natural-cycle transfers, though evidence quality was low to very low. (Busnelli et al., Human Reproduction, 2022) [4]
Key takeaways
Embryo freezing stores fertilized eggs for future use; egg freezing stores unfertilized eggs. [1]
Most clinics now use vitrification, a fast-freezing method. [1]
A frozen embryo keeps the biological age it had when it was frozen. [1]
Freezing and thawing can damage some embryos, and not every embryo survives. [1]
Unused embryos can be discarded or donated to another person, to research, or for training. [1]
What is embryo freezing?
Embryo freezing is the process of freezing and storing embryos so they can be used later. [1] An embryo is an egg that a sperm has fertilized. The process is one of the tools fertility teams use to help people build a family on their own timeline. [1]
It is often called embryo cryopreservation. "Cryo" means cold, and "preservation" means keeping something safe over time.
Why would someone freeze an embryo?
People freeze embryos to keep extra embryos from fertility treatment or to protect their fertility for the future. [1] Treatments like in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) fertilize eggs in a lab and sometimes create more embryos than can be transferred at once. [1]
You may choose to freeze extra embryos if you: [1]
Need to postpone or cancel a transfer after the eggs are already fertilized.
Want to delay IVF to a later date.
Want a backup in case early treatment attempts don't work.
Prefer to donate unused embryos to others or to research rather than destroy them.
Embryo freezing is also used for fertility preservation. For example, a woman with cancer may freeze embryos before chemotherapy or radiation therapy that could affect her ability to get pregnant. [1]
Who can benefit from embryo freezing?
Embryo freezing can help people who face a barrier to pregnancy now but hope to conceive later. [1]
Situation | Why freezing helps |
Advancing age | Embryos keep the age they had when frozen |
Saves extra embryos from one treatment cycle | |
Education or career plans | Allows pregnancy to be delayed for several years |
Cancer treatment | Protects fertility before chemotherapy or pelvic radiation |
Military deployment | Keeps options open during time away |
No partner yet | Embryos can be created with donor sperm |
Source for table: [1]
What is the difference between egg freezing and embryo freezing?
Embryo freezing stores an egg that has already been fertilized, while egg freezing stores an unfertilized egg. [1] Many fertility programs offer both options. [1]
Embryo freezing | Egg freezing | |
What is frozen | Fertilized egg (embryo) | Unfertilized egg |
Sperm needed at freezing | Yes (partner or donor) | No |
Next step after thawing | Transfer to the uterus | Fertilization, then transfer |
What happens before embryo freezing?
Before embryo freezing, you sign consent forms and choose the stage at which the embryos will be frozen. [1] Your permission is required, and the paperwork should explain: [1]
How many embryos will be frozen.
How long they will be stored (often 10 years).
What happens when the storage time ends.
What happens if you die or become too ill to make decisions.
What the embryos may be used for, such as only your own treatment, research, or donation to another couple.
Embryos can be frozen at two main stages: [1]
Stage | Timing after fertilization | Number of cells |
Cleavage stage | About 72 hours (day 3) | 4 to 8 cells |
Blastocyst stage | 5 to 7 days | 200 to 300 cells |
Your fertility team can help you decide which stage is best for your embryos. [1]
How are embryos frozen?
Embryos are frozen with one of two methods: vitrification or slow freezing. [1]
Vitrification is the main method today. The team adds a cryoprotective agent (CPA), a liquid that works like antifreeze and protects cells from ice crystals. The embryos then go straight into tanks of liquid nitrogen at −196 °C (−321 °F). [1]
Slow freezing has fallen out of favor, but some specialists still use it. Embryos get a smaller amount of CPA, and a machine lowers their temperature slowly over about two hours before they move to liquid nitrogen storage. [1]
Vitrification | Slow freezing | |
Cryoprotectant | Larger amount | Smaller amount |
Cooling | Immediate plunge into liquid nitrogen | Gradual cooling over about 2 hours |
Storage | Liquid nitrogen, −196 °C | Liquid nitrogen, −196 °C |
Use today | Standard method | Less common |
Sources for table: [1], [3]
With either method, embryos are stored in small straw-like containers labeled with details that identify them as yours. [1] A frozen embryo does not age in storage. If you freeze an embryo at 35 and use it at 50, it still has the biological age it had at freezing. [1]
What happens when frozen embryos are thawed?
When frozen embryos are needed, a fertility specialist warms them and prepares them for transfer. [1] The steps are: [1]
Remove the embryos from liquid nitrogen.
Let them return slowly to normal temperature.
Soak them to remove the cryoprotective agent.
Use them as planned, usually by transferring them into the uterus.
Does freezing damage embryos?
Freezing and thawing can damage embryos, and some or all frozen embryos may not survive. [1] Your healthcare provider will tell you whether your embryos are suitable for freezing, thawing, and transfer. [1] Research shows vitrified embryos survive thawing more often than slow-frozen ones. [3]
What are the risks of embryo freezing?
The main risks of embryo freezing are damage to embryos, failure to get pregnant after transfer, and some pregnancy complications. [1]
Risk | What it means |
Embryo damage | Freezing or thawing can harm embryos |
Non-viable embryos | Some embryos are not suitable to freeze |
No pregnancy | Transfer may not lead to pregnancy |
Pregnancy complications | Higher rates of preeclampsia and placenta accreta spectrum |
Multiple pregnancy | Twins or triplets if more than one embryo is transferred |
Source for table: [1]
Embryo freezing does not raise the risk of birth defects or health problems in the baby. [1] Outcome studies of frozen-thawed embryos actually show lower rates of preterm birth, low birth weight, growth restriction, and death around the time of birth. [1]
Is IVF more successful with frozen embryos?
Experts do not yet agree on whether fresh or frozen embryos give a better chance of pregnancy. [1] A large review of randomized trials found little or no difference in the overall chance of a live birth between freezing all embryos and a conventional fresh transfer. [2]
Is frozen embryo transfer successful?
Frozen embryo transfer, where a thawed embryo is placed in the uterus, is often successful, but results vary widely. [1] Factors that affect success include: [1]
Both parents' overall health.
The mother's age when her eggs were retrieved.
Fertility problems such as endometriosis, fibroids, and uterine polyps.
Past success or failure of fertility treatments and pregnancies.
Your healthcare provider can explain how these factors apply to you. [1]
Is embryo freezing worth it?
Whether embryo freezing is worth it is a personal decision based on your goals, costs, ethics, and your partner's wishes. [1] Costs vary widely, and medical insurance may not cover fertility treatment. [1]
What if you change your mind about frozen embryos?
You or your partner can change your mind at any time during the process. [1] If one partner decides not to go ahead before freezing, the specialist legally cannot continue. [1] If one partner withdraws after embryos are frozen, a waiting period may be suggested; the embryos are then taken out of storage and allowed to perish. [1]
What can you do with unused frozen embryos?
You can discard unused frozen embryos or donate them. [1] Your options include: [1]
Discard them. The clinic thaws them, and they are no longer viable.
Donate to another person who is trying to have a baby.
Donate to research.
Donate for education, such as training future fertility professionals.
Conclusion
Embryo freezing lets you pause your fertility journey without losing the embryos you have already created. Vitrification has made freezing more reliable, and a frozen embryo keeps the age it had on the day it was frozen. The process still carries real limits, including embryo loss during thawing and higher rates of some pregnancy complications, so the decision deserves a careful talk with your care team.
Considering embryo freezing? Speak with your gynecologist or a fertility specialist about your goals, your embryos, and the consent choices ahead. To understand the full treatment path, read our IVF patient guide next, and bookmark rinnit.com for clear, medically reviewed health guides.
FAQ
What is embryo freezing?
Embryo freezing, or embryo cryopreservation, is a process that freezes and stores fertilized eggs for later use. It is often used after IVF or ICSI and can help people preserve fertility for a future pregnancy. [1]
How long can embryos stay frozen?
Consent forms often set a storage period of 10 years. The forms also explain what happens to the embryos when that storage time ends. [1]
Do frozen embryos age?
No. A frozen embryo keeps the biological age it had at freezing, so an embryo frozen at 35 is unchanged if it is used at 50. [1]
What temperature are embryos stored at?
Embryos are stored in tanks of liquid nitrogen at −196 °C (−321 °F). They sit in small, labeled, straw-like containers. [1]
Is embryo freezing safe for the baby?
Embryo freezing does not raise the risk of birth defects or health problems in the baby. Studies of frozen-thawed embryos show lower rates of preterm birth and low birth weight. [1]
What is the success rate of frozen embryo transfer?
Frozen embryo transfer is often successful, but rates vary widely. Success depends on both parents' health, the mother's age at egg retrieval, fertility problems, and past treatment results. [1]
Can embryos die during freezing or thawing?
Yes. Freezing and thawing can damage embryos, and some or all of them may not survive the process. [1]
What happens to unused frozen embryos?
Unused frozen embryos can be discarded or donated to another person, to research, or for education. The choice is set out in your consent forms. [1]
What is the difference between egg freezing and embryo freezing?
Egg freezing stores unfertilized eggs, while embryo freezing stores eggs that have already been fertilized by sperm. Embryo freezing needs sperm from a partner or donor at the time of freezing.
What is vitrification?
Vitrification is a very fast freezing method that most clinics now use. It cools embryos so quickly that damaging ice crystals are less likely to form.
Related Reading
In Vitro Fertilization (IVF): A Comprehensive Patient Guide walks through each step of IVF and what to expect.
Understanding Fertility Preservation: Options and Benefits compares egg, sperm and embryo freezing to protect future fertility.
Intracytoplasmic Sperm Injection (ICSI): A Targeted Approach to Fertility explains how a single sperm is injected into an egg.
Female Infertility: Causes, Signs, and Treatment Options Explained covers common causes of female infertility and treatment options.
References
Cleveland Clinic. Embryo Freezing (Cryopreservation): Purpose & Results. Cleveland Clinic. Last updated February 17, 2022. https://my.clevelandclinic.org/health/treatments/15464-embryo-freezing-cryopreservation
Zaat T, Zagers M, Mol F, et al. Fresh versus frozen embryo transfers in assisted reproduction. Cochrane Database Syst Rev. 2021;2(2):CD011184. PMID 33539543. https://doi.org/10.1002/14651858.CD011184.pub3
Loutradi KE, Kolibianakis EM, Venetis CA, et al. Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis. Fertil Steril. 2008;90(1):186-193. PMID 17980870. https://doi.org/10.1016/j.fertnstert.2007.06.010
Busnelli A, Schirripa I, Fedele F, et al. Obstetric and perinatal outcomes following programmed compared to natural frozen-thawed embryo transfer cycles: a systematic review and meta-analysis. Hum Reprod. 2022;37(7):1619-1641. PMID 35553678. https://doi.org/10.1093/humrep/deac073
Human Fertilisation and Embryology Authority. Embryo freezing. HFEA. Published May 13, 2016. https://www.hfea.gov.uk/treatments/fertility-preservation/embryo-freezing/
Mancuso F, Singh M, Shanks AL. In Vitro Fertilization. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; last updated April 12, 2026. https://www.ncbi.nlm.nih.gov/books/NBK562266/
Medical disclaimer
This article is for general education and does not replace advice from a qualified healthcare professional. Decisions about fertility treatment, embryo freezing, and embryo transfer should be made with your own doctor or fertility specialist, who knows your full medical history.
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