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In the effectiveness of the IVF program, preparation for frozen embryo transfer (FET) requires not only the correct endometrial thickness but also adequate levels of hormones — estrogen and progesterone — to ensure optimal embryo implantation and support pregnancy development.
HRT-FET (Hormone Replacement Therapy – Frozen Embryo Transfer) — is a protocol for preparing the endometrium for frozen embryo transfer using hormone replacement therapy.
In simple terms:
In a natural cycle, the endometrium prepares for implantation thanks to the woman’s own hormones (estrogen and progesterone), which are produced after ovulation in the corpus luteum of the ovary. But in some cases, this is insufficient or the natural cycle cannot be relied upon — then the reproductive specialist creates a “simulated cycle” using medications.
How HRT-FET (frozen embryo transfer in a hormone replacement cycle) is performed:
- Estrogens (for example, estradiol in tablets or transdermal gel) are started from day 2–5 of the cycle to build up the endometrium.
- When the endometrium reaches the required thickness (usually ≥7–8 mm) and a trilaminar pattern — progesterone is added (in the form of vaginal capsules, injections, etc.).
- After several days from the start of progesterone intake (on the fifth–sixth day), corresponding to the “implantation window,” the embryo is transferred into the uterine cavity.
- After embryo transfer, hormonal support is continued until pregnancy is confirmed, and afterwards — until 12 weeks of pregnancy (placenta formation period).
Who is it suitable for:
- Women with irregular cycles or absence of cycles (e.g., amenorrhea)
- Women over 40, in perimenopause or menopause (with early ovulations, when the endometrium does not have time to form properly)
- Cases where full control of the hormonal environment is needed (e.g., suspected immunological infertility factor)
- When synchronization of donor and recipient cycles is required
Advantages:
- Full control of the cycle
- Ability to schedule embryo transfer
- Good reproductive outcomes in terms of pregnancy rates
In the study by Labarta et al. (2021) (patients undergoing programmed HRT-FET (Hormone Replacement Therapy – Frozen Embryo Transfer) with only vaginal progesterone supplementation), it was found: at progesterone levels < 8.8 ng/ml on the day of embryo transfer — the live birth rate was 35.5% versus 52% in patients with ≥8.8 ng/ml; and the miscarriage risk was 23% versus 13.5%.
Other studies report that the lower threshold of progesterone for embryo transfer in HRT-FET cycles is approximately 10 ng/ml, below which pregnancy rates significantly decline [2], [3].
Labarta et al. (2017) also confirmed reduced ongoing pregnancy rates in cycles with low progesterone during embryo transfer with donor embryos [4], [5].
Approximate progesterone range for programmed FET
| Indicator | Recommended progesterone range (ng/ml) |
| Lower limit | ~8.8–10 ng/ml (minimum critical progesterone level) |
| Optimal range | ~10–20 ng/ml |
| Upper limit |
above 40 ng/ml (possible reduced effectiveness) |
The exact upper threshold is not established, so caution is required!
The role of estrogen (E₂) in preparing the endometrium for frozen embryo transfer
In HRT-FET preparation, estradiol is used to stimulate endometrial growth and prepare it for progesterone exposure (usually up to a thickness of ≥7–8 mm).
Serum estrogen levels (pg/ml) often correlate with endometrial thickness: levels above 180–200 pg/ml are considered sufficient, although the direct correlation with live birth rates (LBR) is weaker compared to progesterone ([7], [8]).
Frequently asked questions about progesterone, estrogen, and the effectiveness of frozen embryo transfer (FET) in IVF programs:
What progesterone level is considered low on the day of embryo transfer?
In most programmed (HRT) cycles, a level <8.8–10 ng/ml on the day of embryo transfer is considered too low and may significantly reduce implantation chances and increase early miscarriage risk.
What to do if progesterone is low on the day of embryo transfer (ET)?
If progesterone is insufficient, the reproductive specialist may:
– postpone the embryo transfer date;
– add intramuscular or subcutaneous progesterone;
– in some cases — switch from vaginal route to a combined approach.
Important: this decision must be made by the reproductive specialist, based on your cycle and preparation protocol.
What progesterone level is considered “too high”?
In some studies, progesterone levels >40 ng/ml before embryo transfer were associated with reduced pregnancy rates (likely due to disruption of synchrony between endometrium and embryo).
However, a clearly defined “upper limit” has not yet been established, and this threshold is approximate.
Should every woman have her progesterone checked before frozen embryo transfer (FET)?
In programmed FET cycles (HRT), where there is no ovulation, checking serum progesterone (P4) before embryo transfer is mandatory.
This allows timely adjustment of support and improves implantation chances.
Can endometrial thickness alone be used without hormone testing?
No. Endometrial thickness is important, but without adequate hormone levels (especially progesterone), even an “ideal” endometrium cannot provide optimal implantation conditions. It is specifically the optimal progesterone level that affects immunological “tolerance” of the endometrium to successful implantation.
Can progesterone be raised using only vaginal suppositories?
In most cases — yes. But:
some women poorly absorb progesterone through the vaginal mucosa (individual absorption, microbiota issues);
in such cases, additional systemic administration (IM or subcutaneous) is recommended.
What is the minimum estrogen level sufficient for frozen embryo transfer (FET)?
Approximately — 180–200 pg/ml or higher.
It is also important that endometrial thickness before starting progesterone is ≥7–8 mm with a trilaminar pattern.
Excessively high estradiol (>1500 pg/ml) is sometimes associated with reduced implantation quality, but no direct link has been proven.
How long should progesterone act before embryo transfer?
Embryos must enter the endometrium within a strictly defined timeframe — the “implantation window.”
For example:
for transferring a day 5–6 blastocyst — 5 full days must pass from starting progesterone to performing embryo transfer (120–144 hours).
Timing is critical — both premature and delayed progesterone exposure may reduce implantation chances.
Which form of progesterone is best: vaginal, injectable, or combined?
The choice depends on individual factors:
Vaginal progesterone — convenient, but may not always provide adequate serum levels.
Injections (IM or subcutaneous) — raise serum progesterone more consistently.
Combined regimens are often used to improve embryo transfer and implantation outcomes (especially with low levels).
How long should progesterone be continued after FET?
Usually — until 11–12 weeks of pregnancy or until the placenta fully takes over hormone production.
The reproductive specialist may gradually taper the support depending on the protocol.
Practical recommendations:
- Mandatory progesterone level monitoring before frozen embryo transfer.
- Critical minimum progesterone cut-off level ≥ 8.8–10 ng/ml on the day of embryo transfer in programmed FET cycles.
- If level <8.8–12 ng/ml — it is advisable to add support (intramuscular or subcutaneous progesterone) individually — this normalizes live birth rates (LBR) in women with low progesterone.
- Optimal range — ~12–20 ng/ml, while avoiding excessively high values (>40 ng/ml), which may reduce implantation efficiency.
- Estrogen level serves more as a marker of endometrial readiness for progesterone exposure, and although strict “target” ranges are lacking, values >180–200 pg/ml are usually sufficient with thickness ≥7–8 mm and trilaminar structure before progesterone initiation.
- Maintaining adequate progesterone levels after embryo transfer during luteal support is essential for successful pregnancy development — women with ongoing pregnancies typically had higher levels up to 12 weeks of pregnancy after ET.
1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10249325/ “Preparation of the endometrium for frozen embryo transfer: an update on clinical practices – PMC”
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC7468003/ “Serum progesterone levels on day of embryo transfer in frozen embryo transfer cycles—the truth lies in the detail – PMC”
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC8902977/ “Impact of endometrial preparation on early pregnancy loss and live birth rate after frozen embryo transfer: a large multicenter cohort study (14 421 frozen cycles) – PMC”
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC8608954/ “Progesterone and estrogen levels are associated with live birth rates following artificial cycle frozen embryo transfers – PMC”
5. https://www.fertstert.org/article/ “Systemic progesterone optimizes programmed frozen embryo transfer outcomes: the only Level I evidence still indicates intramuscular administration” – Fertility and Sterility
