In the early years of IVF, almost all embryo transfers were "fresh" — embryos were transferred 3–5 days after egg retrieval, before any freezing occurred. Today, frozen embryo transfers (FET) account for the majority of IVF transfers in the United States, and in many clinics, fresh transfers have become the exception rather than the rule.
This shift reflects important advances in embryo freezing technology, growing evidence that a frozen cycle can produce equivalent or better outcomes than a fresh transfer, and the clinical benefits of separating the stimulation cycle from the transfer cycle. This guide explains everything you need to know about FET — the protocols, the science, the success rates, and the decisions involved.
What Is a Frozen Embryo Transfer?
A frozen embryo transfer (FET) is an IVF procedure in which a previously vitrified (flash-frozen) embryo is thawed and transferred into the prepared uterus. The embryo may have been frozen days, months, or even years earlier.
The freezing process uses vitrification — an ultra-rapid freezing technique that converts cells to a glass-like state before ice crystals can form. Ice crystal formation was the primary cause of embryo damage with older slow-freeze techniques. Vitrification has transformed FET outcomes: survival rates for vitrified blastocysts now exceed 95% at most high-quality laboratories.
FET vs Fresh Transfer — Key Differences
| Factor | Fresh Transfer | Frozen Embryo Transfer |
|---|---|---|
| Timing | 3–5 days after retrieval | Weeks to months after retrieval |
| Uterine environment | Affected by stimulation hormones | Natural or medicated — more physiologic |
| OHSS risk | Present | Eliminated |
| Cycle coordination | Simpler, fewer procedures | Requires separate cycle |
| Cost | Included in base IVF fee | $3,000 – $5,000 additional |
| Success rates | Comparable or slightly lower | Equal or superior in most studies |
| PGT-A compatibility | Possible but logistically complex | Standard — time to test before transfer |
Why "Freeze All" Is Growing
The strategy of freezing all embryos from a retrieval cycle and delaying all transfers to subsequent FET cycles — called the "freeze-all" or "elective freeze-all" strategy — has grown significantly in recent years. Multiple lines of evidence support it.
The endometrium during stimulation is suboptimal. The high estrogen and progesterone levels produced by multiple follicles during stimulation alter endometrial gene expression. Research published in the New England Journal of Medicine (Chen et al., 2016) demonstrated that in women with PCOS, fresh transfer resulted in significantly lower live birth rates and higher OHSS rates compared to freeze-all with delayed FET.
OHSS elimination. In a stimulated fresh transfer cycle, pregnancy dramatically worsens OHSS because the embryo produces hCG, which continues to stimulate already-hyperstimulated ovaries. Freeze-all eliminates this risk entirely.
PGT-A requires freeze-all. If you're doing genetic testing, embryos must be biopsied and frozen while waiting for genetic results. This inherently requires FET.
Accumulating embryos for single patients. Older patients or those with diminished ovarian reserve may benefit from multiple retrieval cycles to bank embryos before beginning transfers.
For most patients — particularly high responders and those doing PGT-A — freeze-all followed by FET is now considered best practice. For patients with a predicted good uterine environment, one mature follicle, and no OHSS risk, a fresh transfer remains appropriate.
FET Protocols: Natural vs Medicated
There are two main approaches to preparing the uterine lining for FET.
Natural Cycle FET
In a natural cycle FET, your body's own hormones do the work. You begin cycle monitoring on day 2–3 of your period with bloodwork and ultrasound. As the cycle progresses, you monitor for the natural LH surge (via blood test or urine ovulation predictor kit). Once the LH surge is detected, the transfer is timed accordingly — typically 5–7 days after the LH peak for a blastocyst transfer.
Advantages:
- No medications needed (except sometimes low-dose progesterone or vaginal suppositories as support)
- More physiologic — the endometrium develops under your own hormonal signals
- Less expensive
- No estrogen side effects
Disadvantages:
- Requires frequent monitoring at unpredictable times
- Cycle can be cancelled if ovulation doesn't occur or timing is off
- Not feasible for women with irregular cycles or who don't ovulate
- Less control over exact transfer timing
Medicated FET
In a medicated (artificial hormone replacement) FET, external estrogen is used to build the uterine lining, followed by progesterone to synchronize the endometrium with the embryo's developmental age.
Typical medicated FET timeline:
| Day | Event |
|---|---|
| Day 2–3 of period | Start estradiol (pills, patches, or injection) |
| Day 10–14 | Lining check ultrasound; target ≥7mm trilaminar |
| Day 14–16 | Add progesterone (IM or vaginal) |
| Day 19–21 | Blastocyst transfer (5 days after starting progesterone) |
| Day 28–32 | Beta hCG blood test |
Advantages:
- Full scheduling control — transfer date can be planned weeks in advance
- Works for women who don't ovulate
- Consistent hormone levels
- More flexible for work and travel schedules
Disadvantages:
- Daily medications (progesterone often intramuscular)
- Estrogen-related side effects (headaches, breast tenderness, bloating)
- Luteal phase support must continue until ~10 weeks if pregnancy occurs
Progesterone Timing — Why Precision Matters
The most critical variable in FET success is the synchronization between the embryo's developmental stage and the progesterone-primed endometrium. This concept is called endometrial receptivity.
A blastocyst is 5–6 days old at transfer. The endometrium needs to have been exposed to progesterone for exactly 5–6 days (the "window of implantation") for optimal receptivity. Starting progesterone too early or too late misses this window.
In medicated FET cycles, the standard 5-day progesterone-to-transfer interval works for the majority of patients. However, some patients have a displaced window of implantation — they need more or fewer days of progesterone before the endometrium is receptive.
ERA Testing — Is It Worth It?
The Endometrial Receptivity Analysis (ERA) test is a biopsy of the uterine lining taken after the standard progesterone protocol, which is then analyzed by RNA sequencing to determine whether the endometrium is in its receptive state ("pre-receptive," "receptive," or "post-receptive").
If ERA shows a displaced window, the physician adjusts the progesterone timing in the actual FET cycle — for example, adding 12 or 24 hours of progesterone before transfer.
Who ERA is recommended for:
- Two or more unexplained failed FET cycles with good-quality embryos
- Patients with suspected endometrial issues
Limitations of ERA:
- Adds $800–$1,500 to cycle cost
- Requires an additional mock cycle before the actual FET
- Evidence for routine use in unselected patients is mixed; some randomized trials have not shown benefit in patients without recurrent implantation failure
ERA is not currently recommended as a routine test for all FET patients — but for those with recurrent implantation failure, it may identify a correctable timing issue.
Exploring Options Before FET?
Before reaching the frozen embryo transfer stage of treatment, many people wonder if there are simpler, lower-cost options to try first.
MakeAMom makes reusable at-home insemination kits designed for a range of situations: the CryoBaby for frozen or low-volume sperm, the Impregnator for low-motility sperm, and the BabyMaker for those with vaginal sensitivities. Kits cost a fraction of clinical treatment and ship discreetly.
Explore home insemination kits at MakeAMom →
FET Success Rates
FET success rates depend on embryo quality, patient age at the time of retrieval, whether the embryo was tested with PGT-A, and the quality of the endometrial preparation.
FET Success Rates by Age at Egg Retrieval (Own Eggs)
| Age at Retrieval | Live Birth Rate Per FET |
|---|---|
| Under 35 | 45–55% |
| 35–37 | 38–48% |
| 38–40 | 28–38% |
| 41–42 | 18–25% |
| Over 42 | 8–15% |
Note: These rates reflect transfer of good-quality blastocysts. Rates for untested embryos will vary based on the proportion that are chromosomally normal.
FET Success Rates with PGT-A Tested Euploid Embryos
| Age at Retrieval | Live Birth Rate Per Euploid FET |
|---|---|
| Under 35 | 55–65% |
| 35–37 | 55–65% |
| 38–40 | 55–65% |
| 41–42 | 50–60% |
| Over 42 | 45–55% |
The remarkable feature of euploid FET success rates is that they are largely age-independent at the transfer stage — because PGT-A has already selected chromosomally normal embryos. The age effect is captured at the point of retrieval (fewer euploid embryos are obtained from older patients), not at transfer.
To understand how embryo grading affects these outcomes, see our embryo grading and PGT-A testing guide.
Thaw Survival Rates
With modern vitrification, blastocyst survival rates exceed 95% at accredited reproductive laboratories. This means the vast majority of embryos frozen at the blastocyst stage will survive the thaw process intact.
Survival is assessed immediately after thawing by the embryologist. An embryo that has lost <50% of its cells is typically still considered viable for transfer, as blastocysts are remarkably capable of re-expanding after warming. An embryo that has fully degenerated (all cells lost) is unfortunately not transferable, and your physician will advise on next steps.
FET After Failed Fresh Transfer
A failed fresh transfer does not mean FET will fail. In fact, many patients who had an unsuccessful fresh transfer go on to achieve pregnancy with a subsequent FET from the same retrieval cycle. The endometrium — rested and re-prepared without the influence of stimulation hormones — may respond differently.
After a failed transfer, your physician will typically recommend:
- A consultation to review cycle details
- Possibly additional uterine investigation (saline sonogram, hysteroscopy)
- Adjustment of the FET protocol
- ERA testing if there have been multiple failures
- PGT-A testing of remaining embryos if not previously done
For patients with PCOS who had a fresh transfer failure, the freeze-all FET approach is strongly supported by evidence — PCOS is directly associated with endometrial dysfunction during stimulated cycles. See our PCOS and IVF guide.
How Long Can Embryos Stay Frozen?
Vitrified embryos can remain frozen essentially indefinitely. The longest documented successful pregnancy from a frozen embryo involved an embryo stored for over 27 years. Annual storage costs run $500–$1,000, making long-term banking financially feasible.
From a legal and ethical standpoint, most clinics require annual renewal of consent forms for continued storage. If a couple separates, divorces, or one partner dies, the disposition of frozen embryos must follow the consent agreements signed at the time of freezing.
Frequently Asked Questions
Q: Why have frozen embryo transfers become more common than fresh transfers? A: Multiple lines of evidence support the "freeze-all" approach. Research published in the New England Journal of Medicine (Chen et al., 2016) showed that women with PCOS had significantly lower live birth rates and higher OHSS rates with fresh transfer compared to freeze-all with delayed FET. The high estrogen and progesterone environment of a stimulated cycle alters endometrial gene expression in ways that can reduce receptivity. Freeze-all also eliminates OHSS risk, allows time for PGT-A genetic testing, and enables embryo banking across multiple retrieval cycles.
Q: What is the difference between natural cycle FET and medicated FET? A: In a natural cycle FET, your body's own hormones prepare the endometrium, monitoring begins on day 2–3, and transfer is timed around the natural LH surge. It requires no medications (or only light progesterone support) and is more physiologic. In a medicated FET, external estrogen builds the lining over 10–14 days, then progesterone is added for 5–6 days before blastocyst transfer. Medicated FET offers full scheduling control and works for women who don't ovulate, but requires daily medications and continued luteal support through approximately 10 weeks if pregnancy occurs.
Q: How does progesterone timing affect FET outcomes? A: Progesterone timing is the most critical variable in FET success. A blastocyst is 5–6 days old at transfer, and the endometrium must have been exposed to progesterone for exactly 5–6 days — the "window of implantation" — for optimal receptivity. Some patients have a displaced window and need more or fewer days of progesterone before the endometrium becomes receptive. The ERA (Endometrial Receptivity Analysis) test can identify this displacement in patients with recurrent FET failures.
Q: Do euploid embryo transfer success rates still decline with age? A: No — this is one of the most clinically important features of PGT-A tested euploid FET. Success rates per euploid transfer run approximately 55–65% across all age groups from under 35 to over 42, with only modest decline at very advanced ages. The age effect in IVF is primarily captured at egg retrieval (older patients produce fewer euploid embryos from each retrieval), not at the transfer stage once a chromosomally normal embryo is selected.
Q: How long can vitrified embryos remain in storage? A: Vitrified embryos can remain frozen essentially indefinitely. The longest documented successful pregnancy involved an embryo stored for over 27 years. Annual storage costs typically run $500–$1,000. With modern vitrification, blastocyst survival after thaw exceeds 95% at accredited laboratories, and embryos that have lost less than 50% of their cells are generally still considered viable for transfer.
Key Takeaways
- Frozen embryo transfer has become the standard in IVF, with success rates equal or superior to fresh transfer
- Vitrification ensures >95% blastocyst survival after thaw
- Natural cycle FET is appropriate for women with regular ovulatory cycles; medicated FET offers scheduling flexibility
- Progesterone timing is the most critical variable — even a 24-hour shift can affect outcomes
- ERA testing is useful after recurrent FET failure but is not routinely necessary for all patients
- With PGT-A tested euploid embryos, FET success rates are largely age-independent at the transfer stage
This article is for educational purposes only. Individual FET protocols should be designed by a board-certified reproductive endocrinologist based on your clinical history.
