A frozen embryo transfer (FET) is a procedure in which one or more embryos that were frozen during a previous IVF stimulation cycle are thawed and transferred to the uterus. FET cycles are now more common than fresh transfers in the UK: many clinics routinely freeze all embryos from a stimulated cycle and plan a separate transfer in a subsequent month.
This guide explains the two main FET protocols, what the process involves, how costs compare with a full IVF cycle, and what the evidence shows about success rates.
Why Embryos Are Frozen
Embryos may be frozen for several reasons:
Surplus embryos after a fresh transfer. If more good-quality embryos developed than were transferred, the extras are frozen for potential future use. A patient who does not conceive with the fresh transfer can attempt an FET without repeating the stimulation phase.
Freeze-all strategy. Some clinics freeze all embryos from a stimulated cycle rather than doing an immediate fresh transfer. This is recommended when OHSS risk is elevated (particularly for PCOS patients — see PCOS and IVF), when the uterine lining is suboptimal after stimulation, or as a standard approach based on evidence that transfer in a separate unstimulated cycle may improve implantation rates.
Genetic testing. If preimplantation genetic testing (PGT) is being done, all embryos must be biopsied and frozen while results are awaited. The tested, normal embryos are then transferred in a subsequent FET cycle.
Egg freezing to embryo. When previously frozen eggs are thawed, fertilised, and cultured to blastocyst, the resulting embryos may themselves be frozen before transfer.
The Two Main FET Protocols
Natural Cycle FET
In a natural cycle FET, transfer is timed around the patient's natural ovulation. The clinic monitors the cycle with blood tests and ultrasound scans, identifies the moment of ovulation (the LH surge), and schedules transfer approximately 5–6 days later (matching the stage of the endometrium to the blastocyst age).
Advantages: No stimulation medication. Lower medication cost and burden. Closer to a natural physiological state.
Disadvantages: Requires predictable ovulation, so not suitable for patients with irregular cycles or anovulation (including many with PCOS). Cycle timing is less controllable, which can create scheduling difficulties.
Modified Natural Cycle FET
A variation of the above where a low-dose hCG trigger injection is given at the time of the natural LH surge to optimise timing. Otherwise similar to a natural cycle.
Medicated (Artificial) Cycle FET
In a medicated FET, the natural cycle is suppressed (sometimes, not always) and the endometrium is prepared using exogenous oestrogen tablets or patches, followed by progesterone. Transfer is scheduled at a fixed point after progesterone starts, giving the clinic full control over timing.
Advantages: Precise scheduling. Suitable for patients with irregular cycles. Cycle can be planned around the patient's or clinic's availability.
Disadvantages: Requires oestrogen and progesterone medication for several weeks. Some side effects from the hormone support. Progesterone continues until approximately 10–12 weeks of pregnancy if the transfer is successful.
Most clinics use the medicated protocol as their default for simplicity and scheduling control. The choice of protocol should be discussed with your consultant, as some evidence suggests natural cycles may have marginally better outcomes for patients who ovulate regularly.
What Happens on Transfer Day
FET day is substantially simpler than egg collection day. No sedation is required.
The procedure takes approximately 10–15 minutes. A speculum is placed, the cervix is visualised, and a thin catheter is passed through the cervical canal under abdominal ultrasound guidance. One (or sometimes two) embryos are placed in the uterine cavity.
You will typically rest briefly and then go home. There are no restrictions on eating or drinking beforehand. Progesterone supplementation continues after transfer.
The two-week wait that follows is emotionally demanding for most patients. For an overview of the full process, see IVF timeline: week by week.
FET Costs in the UK
An FET cycle costs significantly less than a full stimulated IVF cycle because the expensive egg collection and laboratory fertilisation steps are not repeated.
| Item | Typical cost | |---|---| | FET clinic fee | £1,200–£2,500 | | Medication (medicated protocol) | £200–£600 | | Monitoring scans | £150–£400 | | Total | £1,500–£3,500 |
Some clinics include a specified number of FET cycles in an initial IVF package; others charge separately. Confirm with your clinic what is included in any package price.
For NHS patients, if frozen embryos are available from an NHS-funded IVF cycle, the FET from those embryos is typically also NHS-funded, subject to ICB policy. Check your ICB's commissioning policy at nestie.co/nhs.
Success Rates: FET vs Fresh Transfer
HFEA data consistently shows that FET live birth rates are comparable to, and in some subgroups higher than, fresh transfer rates. For a full explanation of how to read HFEA success rate data, see HFEA success rates explained.
The reasons FET results compare favourably:
- The uterine environment is less disrupted by stimulation hormones — endometrial receptivity may be better in a natural or artificial cycle than in a cycle where high oestrogen from stimulation is present.
- Blastocyst vitrification survival rates are high (typically 90%+ for good-quality embryos).
- FETs typically use blastocysts — the subset of embryos that survived to Day 5, which are already pre-selected for developmental competence.
One important caveat: FET success rates are not equivalent to a "fresh go" at conception — they represent the expected outcome from embryos already created, which have a defined quality profile. Whether a freeze-all approach is better than fresh transfer is still debated in the literature; the benefit depends on patient subgroup.
Frequently Asked Questions
Q: Does freezing and thawing embryos damage them?
A: Modern vitrification (rapid-freeze) has very high survival rates — typically 90%+ for good-quality blastocysts. The vast majority of clinical experience suggests that embryo quality is well preserved through vitrification. Babies born from frozen embryos have similar health outcomes to those born from fresh transfers.
Q: How long can embryos be stored?
A: In the UK, embryos can be stored for up to 55 years with consent renewal every 10 years. Initially the limit was 5 years, extended to 10 years, and then to 55 years by Parliament in 2022. Storage is charged annually by the clinic — typically £200–£400 per year.
Q: Can I choose which embryo is transferred in an FET?
A: Yes, with your embryologist's input. Embryos are graded when they are frozen, and your clinic will typically propose transferring the highest-graded embryo first. If you have had PGT (preimplantation genetic testing), the choice is guided by the test results.
Q: How many FET cycles does the NHS fund?
A: NHS funding typically covers the FET cycles included as part of the funded IVF treatment — meaning the transfer of embryos created during an NHS-funded stimulated cycle. The specific policy on how many FETs are included varies by ICB. See how many IVF cycles does the NHS fund? for more detail.
Q: Does it matter whether I have a natural or medicated FET?
A: There is some evidence that natural cycle FET may have marginally better outcomes for patients who ovulate regularly, because it avoids the high-progesterone environment created by exogenous supplementation. In practice, many clinics default to medicated cycles for scheduling ease. If you ovulate regularly and your clinic offers both, it is worth asking your consultant which protocol they would recommend for your situation.
This article is for information only and does not constitute medical advice. Clinic protocols and NHS policies vary; always confirm with your clinic and ICB.