Frozen Embryo Transfer (FET): The Complete Guide
A frozen embryo transfer (FET) thaws a cryopreserved embryo and transfers it into the uterus in a separate cycle from egg collection. FET has become the dominant embryo transfer strategy globally — in many cases outperforming fresh transfer.
Why FET Often Outperforms Fresh Transfer
Vitrification technology: Ultra-rapid freezing eliminates ice crystal damage. Vitrified blastocyst survival rates exceed 95% — frozen embryos are essentially as viable as fresh.
Fresh transfer disadvantages: During ovarian stimulation, high estrogen from multiple follicles can create a less receptive endometrial environment than a natural cycle. FET allows the uterus to recover.
OHSS prevention: Women with PCOS or high AMH can use a GnRH agonist trigger and freeze all embryos, eliminating severe OHSS risk.
PGT-A: Embryo genetic testing takes 7–14 days, requiring all tested embryos to be frozen.
Two FET Protocols
Natural Cycle FET (NC-FET) The body's own cycle prepares the endometrium. Ovulation is tracked by LH test or ultrasound; blastocyst transfer is timed 5 days after ovulation.
Best for: Women with regular ovulatory cycles. Advantage: Minimal or no medication; lower cost; evidence suggests slightly lower risk of pregnancy complications (hypertension, large-for-gestational-age).
Medicated (Artificial) FET (M-FET) Estrogen for 12–14 days grows the lining; progesterone triggers the secretory phase; transfer is 5 days after starting progesterone.
Best for: Irregular cycles, PCOS, or when precise scheduling is needed. Disadvantage: Some studies show modestly higher rates of hypertensive disorders in pregnancy compared to natural cycle.
Per ESHRE 2023: Natural cycle FET is preferred for women with regular ovulatory cycles. Medicated FET is equally valid when natural cycle is not feasible.
The FET Process Step by Step
Natural cycle FET:
- Baseline scan (cycle Day 2)
- Monitoring scans from Day 10–12 — follicle size and lining
- LH surge detected (OPK or blood test) or ovulation confirmed by ultrasound
- Transfer on Day 5 after ovulation (for Day 5 blastocyst)
- Progesterone support begins at transfer
- Pregnancy blood test 10–14 days later
Medicated FET:
- Estrogen from Day 1–3 of cycle (2mg × 3/day or patches)
- Lining check at Day 12–14 (target ≥7mm, trilaminar pattern)
- Progesterone begins when lining is adequate
- Transfer on Day 5 of progesterone
- Continue estrogen + progesterone to 10–12 weeks if positive
- Pregnancy test 10–14 days after transfer
How to Prepare for FET
Medications: Take exactly as prescribed — missed progesterone can cause the transfer to fail regardless of embryo quality. Set reminders.
Aspirin 75mg: Commonly prescribed from the start of the cycle to support endometrial blood flow.
Avoid NSAIDs (ibuprofen, naproxen) from the progesterone start date — they impair prostaglandin-mediated implantation.
Transfer day: Arrive with a comfortably full bladder (aids ultrasound guidance). The procedure takes 10–15 minutes. No sedation needed. Resume light normal activity the same day — there is no evidence bed rest improves outcomes.
Alcohol: Abstain from transfer day onwards through the two-week wait.
FET Success Rates (ESHRE 2023, live birth per transfer)
| Age at Egg Collection | Live Birth Rate per FET |
|---|---|
| Under 35 | 40–50% |
| 35–37 | 35–42% |
| 38–40 | 25–35% |
| 41–42 | 15–22% |
| Donor eggs | 40–50% any age |
Why FET Failed: What to Investigate
After 2 failed transfers with good embryos, consider:
- Hysteroscopy — exclude polyps or adhesions
- ERA test — personalise progesterone timing
- Chronic endometritis biopsy — subclinical infection impairs implantation
- Sperm DNA fragmentation — elevated DFI causes early pregnancy loss
Reference: ASRM — Frozen Embryo Transfer, 2023. ESHRE — Natural vs Medicated FET, 2023.
Frequently Asked Questions
Is frozen embryo transfer better than fresh?▾
For most patients, FET achieves equal or slightly higher live birth rates than fresh transfer. Fresh transfer in a stimulated cycle may expose the embryo to a hormonally disrupted uterine environment. The main exception is poor responders with few embryos — for them, fresh transfer avoids the small risk of embryo loss during freezing, though vitrification survival is >95%.
How long does a FET cycle take?▾
The FET procedure itself takes 10–15 minutes and requires no anaesthetic. The preparation takes 4–6 weeks — about 2–3 weeks for a natural cycle FET, or 12–16 days of hormone preparation for a medicated FET. Total from starting hormones to pregnancy test: approximately 4–6 weeks.
What should I do after a frozen embryo transfer?▾
Resume light normal activities. There is no evidence that bed rest improves outcomes. Avoid strenuous exercise, alcohol, and NSAIDs. Continue prescribed progesterone and estrogen (if on medicated protocol) exactly as directed. The beta-hCG pregnancy blood test is done 10–14 days after transfer.