Freezing embryos has moved from a back-up option to a routine part of modern IVF. In the United Kingdom, the fertility regulator reports that frozen embryo transfers made up 48% of all IVF cycles in 2024, double the 24% recorded in 2014. Many patients now meet the idea of freezing early in their treatment: spare embryos may be frozen after a cycle, or every embryo may be frozen deliberately and transferred in a later cycle. Even so, the process is often poorly explained. How are embryos frozen without being damaged? What happens when they are thawed? How long can they stay in storage, and are babies born from frozen embryos as healthy as those born from fresh ones?
This article answers those questions using regulatory information, systematic reviews and randomised trial evidence. It is general education, not advice for any individual, and no freezing method can guarantee that an embryo will survive thawing or lead to pregnancy.
Why embryos are frozen
Embryos may be frozen for several reasons, which can overlap:
Spare embryos: After a fresh transfer, additional good-quality embryos can be stored for later attempts or for a future sibling, so another egg collection is not needed.
Single embryo transfer: Freezing makes it possible to transfer one embryo at a time and still keep other embryos in reserve, which lowers the chance of twins.
Freeze-all cycles: The whole group of embryos is frozen when transfer in the same cycle is not advisable, for example when there is a high risk of ovarian hyperstimulation syndrome, when hormone levels are unfavourable around the time of transfer, or when embryos are waiting for genetic test results.
Medical or personal reasons: Treatment may need to pause because of illness, surgery, or the need to treat a uterine problem first.
Fertility preservation: Embryos may be created and frozen before treatment that could reduce future fertility.
How embryos are frozen
The principal challenge of freezing a living cell is water. If ice crystals form inside an embryo, they can rupture delicate structures. The HFEA, the UK regulator, describes the approach this way: embryos are placed in a cryoprotectant solution that draws water out of the cells and protects them from ice-crystal damage, and are then stored in liquid nitrogen tanks. Embryos can be frozen at the single-cell stage, at the two- to eight-cell stage, or as blastocysts, which are embryos grown for around five to six days. Before freezing, patients are usually screened for infections such as HIV and hepatitis, because storage tanks are shared spaces and laboratories must manage contamination risk.
Two broad methods exist. Slow freezing lowers the temperature gradually using a programmed freezer with lower concentrations of cryoprotectant. Vitrification uses higher concentrations of cryoprotectant and extremely rapid cooling, so that the liquid turns into a glass-like solid without forming ice crystals. The HFEA notes that most embryos are now frozen by vitrification. The word comes from the Latin vitrum, meaning glass.
What the evidence says about vitrification
The question of whether vitrification truly works better than slow freezing has been studied in randomised trials and pooled in systematic reviews.
A systematic review and meta-analysis published in Human Reproduction Update compared the two methods for oocytes (eggs), cleavage-stage embryos and blastocysts. Across seven randomised trials of embryos (3,615 embryos), survival after warming was significantly higher with vitrification (relative risk 1.59). For clinical pregnancy and live birth, results favoured vitrification in randomised trials, but the authors graded this evidence as low quality, because the trials were small and varied from one to another.
A Cochrane review on egg freezing found that vitrification was associated with a higher clinical pregnancy rate than slow freezing, based on two small trials of 106 women, with moderate-certainty evidence for that outcome. Live births and adverse events were not reported in those trials.
The authors of the 2017 meta-analysis concluded that laboratories still using slow freezing should consider moving to vitrification, mainly because of the survival data. For patients, the practical meaning is that modern laboratories have a reliable technique, but outcomes still depend on embryo quality at the time of freezing, the age of the woman who provided the eggs, and the laboratory's own experience and quality control.
What happens at thawing (warming)
When an embryo is needed, it is removed from storage and warmed quickly, and the cryoprotectant is gradually diluted out so that the cells return to their normal state. Embryologists then check the embryo under the microscope for signs of survival, such as intact cells and, in blastocysts, re-expansion of the fluid-filled cavity.
The HFEA explains that not all embryos survive thawing, and very occasionally none do. Surviving embryos sometimes lose a cell or two but often recover and carry on developing. The clinic decides whether a thawed embryo is suitable for transfer. If it is not, patients may be offered another frozen embryo if one is available. These realities are why patients are often advised to keep any extra embryos rather than assuming the first thaw will proceed without difficulty.
Preparing for a frozen embryo transfer
A frozen transfer is a separate treatment step. The lining of the uterus must be ready at the point the embryo is warmed and placed, and this is achieved in one of two main ways. In a natural cycle, the timing follows the woman's own ovulation, which is monitored with ultrasound or blood tests. In a medicated (artificial) cycle, oestrogen is given to build the lining, followed by progesterone to prepare it for implantation, which allows the transfer date to be scheduled. Which approach is appropriate depends on factors such as whether the woman ovulates regularly and what the clinic's protocol and the evidence for the specific situation suggest. This is a clinical decision made with the treating team.
Is a frozen embryo as safe as a fresh one?
The HFEA states that using frozen embryos is as safe as using fresh ones, and identifies multiple birth as the main risk, which is reduced by transferring a single embryo. The picture from randomised trials is more nuanced, and worth understanding.
A 2021 Cochrane review compared a freeze-all strategy with conventional IVF that includes a fresh transfer, using eight trials with 4,712 women in the main analysis. It found probably little or no difference in cumulative live birth. Freeze-all may reduce the risk of ovarian hyperstimulation syndrome (about 3% with conventional treatment compared with roughly 1% with freeze-all). On the other hand, freeze-all may increase the risk of hypertensive disorders of pregnancy and of babies who are large for gestational age, and birth weight was on average higher. The reviewers rated much of the safety evidence as low certainty, and evidence on miscarriage and multiple pregnancy as very low certainty. They also noted that time to pregnancy was probably shorter with conventional treatment.
In practical terms, this means freezing is neither automatically better nor automatically riskier than fresh transfer. Specialists weigh the individual circumstances, such as OHSS risk, hormone levels, genetic testing plans and the patient's own history. Patients who are told that a freeze-all approach is planned can reasonably ask why it is being recommended for them, and what the expected benefit and trade-offs are.
How long can embryos be stored?
Storage limits are set by national law and professional rules, and they vary by country, so patients should check the rules where their treatment takes place. As one example, the HFEA explains that since 1 July 2022 embryos in the UK can be stored for up to 55 years from the date they were first placed in storage, with consent renewed every 10 years. Before that date, storage was usually limited to 10 years, with longer periods only in cases of premature infertility or medical treatment affecting fertility. If consent is not renewed, the material must be removed from storage and disposed of once it can no longer be lawfully stored, which is why clinics need up-to-date contact details.
The HFEA also states that there is not enough evidence to confirm whether longer storage affects embryo viability, while also noting that no evidence has shown that storage length affects embryos. In other words, current evidence has not identified a clear harm from storage time, but long-term data are still limited, particularly beyond a decade or two. The age of the woman at the time the embryo was created is generally considered a stronger determinant of its potential than the length of time spent frozen.
Consent and decisions about unused embryos
Embryo storage involves legal and ethical decisions, not only laboratory ones. In the UK, both the person receiving treatment and the egg or sperm provider must consent to storage, and consent can be varied or withdrawn before the embryos are used. If a partner withdraws consent, the embryos cannot be used in treatment, and there may be a cooling-off period of up to a year when one party withdraws. Consent forms also cover what happens to embryos if a person dies or loses the capacity to decide. Options for unused embryos commonly include continued storage, donation to another person or couple, donation to research or training, or disposal, depending on what local law allows. Counselling is typically offered, and many patients find that their views change over time, which is why decisions on consent are usually revisited at renewal.
Questions worth asking your care team
At what stage will my embryos be frozen, and which freezing method does the laboratory use?
How does the laboratory monitor survival after warming, and what is the usual plan if an embryo does not survive?
Is a natural or medicated cycle suggested for my frozen transfer, and why?
Am I being advised to freeze all embryos, and if so, what are the medical reasons in my situation?
What are the local legal limits and renewal requirements for storage, and what happens if I move or change clinics?
What are my options for any embryos I decide not to use?
Key takeaways
Vitrification has become the standard way to freeze embryos because it avoids ice-crystal formation and, in pooled trial data, improves survival after warming compared with slow freezing. Thawing is a carefully controlled process, but survival is not guaranteed for every embryo. Frozen transfers are a well-established route that can give cumulative outcomes comparable with fresh transfer for many patients, with a lower risk of OHSS, while some pregnancy complications may be more common after freeze-all strategies and are still being studied. Storage rules and consent requirements differ between countries. As with every stage of IVF, the right approach depends on age, diagnosis, embryo quality and individual preferences.
References
- Human Fertilisation and Embryology Authority. Embryo freezing. HFEA, accessed October 2026. https://www.hfea.gov.uk/treatments/fertility-preservation/embryo-freezing/
- Human Fertilisation and Embryology Authority. Consent to treatment and storage. HFEA, accessed October 2026. https://www.hfea.gov.uk/choose-a-clinic/consent-to-treatment-and-storage/
- Human Fertilisation and Embryology Authority. Fertility treatment 2024: trends and figures. HFEA, 2026. https://www.hfea.gov.uk/about-us/publications/research-and-data/fertility-treatment-2024-trends-and-figures
- Rienzi L, Gracia C, Maggiulli R, LaBarbera AR, Kaser DJ, Ubaldi FM, Vanderpoel S, Racowsky C. Oocyte, embryo and blastocyst cryopreservation in ART: systematic review and meta-analysis comparing slow-freezing versus vitrification to produce evidence for the development of global guidance. Human Reproduction Update, 2017;23(2):139-155. https://academic.oup.com/humupd/article/23/2/139/2670008
- Glujovsky D, Riestra B, Sueldo C, et al. Vitrification versus slow freezing for women undergoing oocyte cryopreservation. Cochrane Database of Systematic Reviews, 2014. https://www.cochrane.org/evidence/CD010047_vitrification-comparison-slow-freezing-egg-cryopreservation-women-undergoing-assisted-reproduction
- Zaat T, Zagers M, Mol F, Goddijn M, van Wely M, Mastenbroek S. Fresh versus frozen embryo transfers for assisted reproduction. Cochrane Database of Systematic Reviews, 2021, Issue 2. https://www.cochrane.org/evidence/CD011184_fresh-versus-frozen-embryo-transfers-assisted-reproduction