Frozen Embryo Transfer: Rates, Protocols and Trade-offs

The honest picture, with numbers

Frozen embryo transfer has stopped being the fallback and become the mainstream. The HFEA reports that frozen transfers accounted for almost half — 48% — of all IVF cycles carried out in 2024, up from 24% in 2014. In the 2023 cohort they made up 45% of all embryo transfers.

The headline rates, from the HFEA's 2023 treatment cohort, are these. Using patients' own eggs, the average pregnancy rate per embryo transferred was 39% for frozen transfers and 31% for fresh; the average birth rate per embryo transferred was 33% for frozen and 25% for fresh.

Now the caveat, which the HFEA states itself and which almost every clinic page omits: birth rates from fresh and frozen embryo transfers are not directly comparable. Embryos that survive to be frozen are a selected group — they were good enough quality to be worth storing. Comparing them with all fresh transfers compares a filtered population with an unfiltered one. The gap is partly real and partly selection.

Why so many cycles now end in the freezer

Two things drove the shift. First, vitrification, the fast-freezing technique the HFEA describes as now standard, made survival through the thaw far more reliable. Second, single embryo transfer became normal practice, which by definition leaves embryos over. The HFEA links the rise in frozen transfers directly to the rise in single embryo transfers over the last decade.

There is also a deliberate version of this, in which no fresh transfer is attempted at all and everything is frozen for later. The HFEA rates elective freeze-all as a treatment add-on, and the ratings are worth reading precisely: amber for improving the chance of having a baby in most patients, because the evidence conflicts; green for reducing the risk of ovarian hyperstimulation syndrome, both in most patients and in those at increased risk; and grey for reducing adverse obstetric or neonatal outcomes, because the evidence is insufficient.

Read together, that is a clear message. Freeze-all is a proven tool for avoiding hyperstimulation. It is not a proven tool for having a baby faster.

Natural cycle or medicated: what NICE says

Frozen transfers are prepared in one of two broad ways: tracking your own cycle and timing the transfer to your own ovulation, or suppressing the cycle and building the lining with hormone medication.

NG257 recommendation 1.49.12 advises women, and trans men and non-binary people with female reproductive organs, who have regular ovulatory cycles that the likelihood of a live birth after replacement of frozen-thawed embryos is similar for embryos replaced during natural cycles and hormone-supplemented cycles. If your cycles are regular, in other words, the guideline does not favour one protocol on live birth grounds — so the choice can legitimately turn on monitoring burden, scheduling and side effects.

There is one safety signal worth knowing. The HFEA notes research suggesting that patients who undergo a programmed, medicated frozen transfer rather than a natural cycle may be at increased risk of high blood pressure during pregnancy. That is a reason to ask why a medicated protocol is being chosen if a natural one is available to you.

Whose age counts

A point that causes a lot of unnecessary anxiety: with frozen embryos, the age that matters is your age when the eggs were collected, not your age at transfer. The HFEA states that when using frozen embryo transfers, pregnancy and birth rates are largely related to the age of the patient at egg collection rather than age at embryo transfer, and its own by-age figures for frozen transfers are calculated on age at collection for that reason.

The obstetric trade-off

Frozen and fresh transfers do not produce identical pregnancies. The HFEA summarises the evidence this way: birthweight of babies born from fresh IVF cycles is lower, while birthweight from frozen transfer cycles is higher, and that increased chance of a large-for-gestational-age baby, together with an increased risk of hypertensive disorders such as pre-eclampsia, is why it counsels caution before adopting a freeze-all policy for everyone.

This is not an argument against frozen transfer, which is now the majority of UK practice. It is an argument for knowing that the trade is a different risk profile rather than a free upgrade.

Storage, consent and the thaw

The HFEA is clear that although most embryos survive the freeze-thaw process, some do not, and there is no evidence that embryos are affected by how long they have been frozen. On storage, the law changed on 1 July 2022: embryos can now be stored for use in treatment for any period up to a maximum of 55 years from the date they first went into storage, but consent must be renewed every 10 years for storage to lawfully continue. If your clinic cannot reach you, your embryos are at risk of being removed from storage and disposed of, which makes keeping your contact details current a genuinely consequential piece of admin.

What to do next

1. Ask for the clinic's frozen transfer figures for your age at egg collection, not its overall frozen figure.
2. Ask which protocol is proposed and why. If your cycles are regular, NG257 says live birth is similar either way.
3. Ask whether freeze-all is being proposed for hyperstimulation risk or for success. The HFEA rates it green for the first and amber for the second.
4. Ask what your embryos are graded and how many are stored, and what the annual storage fee is.
5. Diarise your storage consent renewal and update your contact details with the clinic whenever you move.
6. Tell your maternity team you conceived from a frozen transfer if you become pregnant, so blood pressure monitoring is planned accordingly.

When to seek help

Because there is no egg collection in a frozen cycle, the hyperstimulation risk that dominates a fresh cycle is largely absent — that is exactly why freeze-all is rated green for OHSS reduction. Contact your clinic promptly for heavy bleeding, severe one-sided pain, shoulder-tip pain, or fainting, which can indicate an ectopic pregnancy, a risk the HFEA notes applies to IVF pregnancies generally. In pregnancy, report headaches, visual disturbance or sudden swelling to your maternity team without delay, given the raised hypertensive risk associated with medicated frozen transfers.