Frozen Embryo Transfer Preparation: Natural, Modified Natural or Programmed Cycle?

Key Takeaways

A frozen embryo transfer can be prepared with a true natural, a modified natural or a programmed (HRT) cycle. In women who ovulate regularly, 2026 randomized evidence found similar live birth rates, with less pregnancy loss and fewer pregnancy complications such as pre-eclampsia, when ovulation is preserved. A programmed cycle remains the practical choice when ovulation is absent or hard to monitor, or when the date must be tightly controlled.

Key evidence: Wei et al. 2026 — natural ovulation vs programmed FET, randomized trial (BMJ) · Erden et al. 2026 — natural cycle FET, systematic review and meta-analysis (Hum Reprod Update) · Labarta et al. 2021 — serum progesterone on transfer day in HRT cycles (Hum Reprod)

Frozen Embryo Transfer: Natural or Artificial Cycle?

Table of Contents

1. Preparing the Endometrium
2. Why Timing Matters
3. HRT or Programmed Cycles
4. Natural Cycles
5. Hybrid or Modified Natural Cycles
6. How the Choice Is Made
7. Safety and Pregnancy Considerations
8. When Personalization Matters Most
9. Frequently Asked Questions for Dr. Aksoy
10. Sources

Preparing the Endometrium for Frozen Embryo Transfer

A frozen embryo transfer begins with one practical advantage: the embryo is already available. The next task is to make sure the uterine lining and the embryo are ready at the same time.

There is more than one way to do that. There are three main ways to do that: a true natural cycle, which follows the patient’s own ovulation; a modified natural cycle, in which ovulation is usually triggered with an hCG injection; and a programmed cycle (also called a medicated, artificial or HRT cycle), in which estrogen and progesterone replace ovulation. For women who ovulate regularly, the evidence now leans towards keeping ovulation.

Dr. Aksoy’s Approach: In a woman who ovulates regularly, I now generally prefer to preserve ovulation unless there is a clear reason not to. Natural and modified natural FET give reproductive outcomes similar to programmed cycles, while current evidence suggests lower pregnancy loss and fewer hypertensive pregnancy complications (Wei et al., 2026; Martins and Nastri, 2026). I still use programmed cycles when ovulation is absent or difficult to track, or when the clinical and logistical advantages outweigh these differences.

Why Timing Matters So Much

Frozen transfer is, above all, a timing exercise. The endometrium needs to be in the right biological phase when the embryo is transferred.

That means the team considers more than the lining’s appearance. The timing of ovulation or progesterone exposure has to match the developmental stage of the embryo. If those two clocks are out of step, a reassuring scan does not tell the whole story.

HRT or Programmed Cycles

In a programmed cycle (also called a medicated, artificial or HRT cycle), estrogen and progesterone are used to prepare the lining without relying on spontaneous ovulation. This can be helpful when cycles are irregular, ovulation is difficult to predict, or the transfer needs to fit a carefully planned schedule.

The main advantage is control. The trade-off is more medication and a different pregnancy risk profile. A large randomized trial in women who ovulate found more pre-eclampsia, more hypertensive disorders of pregnancy and more postpartum haemorrhage after programmed cycles than after natural ovulation, with similar live birth rates (Wei et al., 2026). Earlier cohort data pointed the same way (Roelens et al., 2022). One proposed explanation is the corpus luteum, the temporary structure that forms in the ovary after ovulation. Natural and modified natural cycles have one; a programmed cycle, without ovulation, does not. Besides estrogen and progesterone, the corpus luteum produces vasoactive substances such as relaxin and vascular endothelial growth factor, which are thought to matter for early placentation and are not replaced in a programmed cycle (Singh et al., 2020). This remains a hypothesis rather than a proven cause. None of this makes HRT unsuitable for everyone; it means the reason for choosing it matters.

Natural Cycles

In a natural cycle, the clinic follows spontaneous ovulation and plans the transfer around the patient’s own biology. For someone who ovulates regularly, this means less medication and a preparation that follows the natural cycle more closely.

The practical side is less predictable. Monitoring may need to be more flexible, and a cycle may be postponed if ovulation cannot be dated with enough confidence. In the randomized trial above, cycles were cancelled more often with natural ovulation than with a programmed regimen (16.2% vs 11.5%) (Wei et al., 2026). “Natural” does not always mean “simpler”; it means that the timing comes from the patient’s cycle rather than from a fully programmed schedule.

Dr. Aksoy’s approach to luteal support: In a true natural cycle the corpus luteum produces progesterone, but I still prefer to add progesterone support in the luteal phase, because randomized trials and meta-analyses have reported higher live birth rates with it in true natural-cycle FET (Erden et al., 2026). I use hCG to time ovulation when needed, not as luteal support. Regular ovulation, the LH and progesterone pattern, how adequate the corpus luteum appears and the natural-cycle protocol we use all change my decision.

Hybrid or Modified Natural Cycles

In a modified natural cycle, the patient grows a follicle and forms her own corpus luteum, as in a natural cycle, but ovulation is usually triggered with an hCG injection so that the transfer date can be planned more predictably. It is a practical middle ground between physiology and scheduling, sometimes called a hybrid cycle. A 2026 meta-analysis of randomized trials found similar live birth rates with modified natural and true natural cycles (Venetis et al., 2026).

This can be useful when ovulation is present but not perfectly predictable, or when a patient wants less medication than a fully programmed cycle. In hCG-triggered modified natural cycles, routine extra progesterone has not been shown to increase live birth: a 2026 randomized trial and a 2026 meta-analysis found no clear benefit (Saupstad et al., 2026; Erden et al., 2026). The exact plan is individualized. A protocol found online cannot replace the monitoring and decisions made by the treating team.

How the Choice Is Made

For women who ovulate regularly, the evidence has moved towards preserving ovulation. In the largest randomized trial so far, 4,376 women aged 20–40 who ovulate were assigned to a natural-ovulation or a programmed regimen before a single frozen blastocyst transfer. Healthy live birth rates were similar (41.6% vs 40.6%), while early pregnancy loss (12.1% vs 15.2%), pre-eclampsia (2.9% vs 4.6% of clinical pregnancies) and postpartum haemorrhage (2.0% vs 6.1% of deliveries) were less common after natural ovulation (Wei et al., 2026). A congress review of ESHRE 2025 concluded that, for women who ovulate regularly, natural-cycle FET “may now be considered the preferred approach” (EMJ congress review, 2025).

A 2026 meta-analysis of 17 randomized trials (10,611 women) found high-certainty evidence of less pregnancy loss when ovulation is preserved (risk ratio 0.75) and similar live birth rates overall; when trials at high risk of bias were excluded, live birth was higher with ovulation. For hypertensive disorders of pregnancy the result pointed the same way but was not statistically certain. The authors advise clinicians to prioritise ovulatory preparation while keeping cycle cancellation low (Martins and Nastri, 2026).

For women who do not ovulate regularly, a programmed cycle remains the practical option. In practice, the discussion usually comes back to five questions:

• Is ovulation regular and possible to monitor reliably?
• How much control over the transfer date is needed?
• Can progesterone timing be aligned with the embryo stage?
• Are there medical or obstetric factors that change the balance?
• Which plan can the patient follow consistently?

The useful question is now: can this patient use an ovulatory protocol safely and reliably, and if not, which programmed approach suits her best?

Safety and Pregnancy Considerations

The choice deserves a closer conversation when the risk of hypertensive disorders of pregnancy is higher. The authors of the randomized trial recommend a natural-ovulation regimen especially for these patients, for example women aged 38 or older and women with obesity (Wei et al., 2026). These are group results; they do not predict the outcome for an individual patient.

This does not turn every HRT cycle into a bad option. It means the risk discussion should be part of the planning, especially when regular ovulation makes a natural or modified natural option feasible.

When Personalization Matters Most

The plan may need more individual thought when cycles are irregular, the lining has been difficult to prepare, or transfers have failed repeatedly. Endometriosis or questions about the endometrial microbiome may also belong in the wider assessment, but neither automatically dictates one transfer protocol.

PCOS does not automatically mean a programmed cycle. What matters is whether ovulation occurs reliably and can be monitored: a woman with PCOS who ovulates regularly may still be suited to an ovulatory protocol, whereas anovulation may make a programmed cycle or ovulation induction more practical.

After repeated failure, a focused assessment is usually more useful than adding tests at random. ESHRE does not recommend the currently available endometrial receptivity tests (ESHRE add-ons recommendations, 2023). If chronic endometritis is being considered, its investigation and treatment should be based on the clinical picture rather than on an automatic change of transfer protocol.

Frequently Asked Questions for Dr. Aksoy

Is HRT or a natural cycle better for frozen embryo transfer?

In a woman who ovulates regularly, I now generally prefer to preserve ovulation, with a natural or modified natural cycle, unless there is a clear reason not to. Current evidence shows live birth rates similar to programmed cycles, with lower pregnancy loss and fewer high blood pressure complications in pregnancy. I still choose a programmed cycle when ovulation is absent or hard to track, or when its clinical and practical advantages outweigh these differences.

When do I start estrogen in an HRT cycle, and when is the transfer done?

Estrogen usually starts on day 2 or 3 of the period, as tablets or patches, once the baseline ultrasound is normal. After about 10–14 days the lining is checked, and if it has developed well, progesterone is started. The transfer day is set by matching progesterone exposure to the embryo’s developmental day: a day-5 blastocyst is usually transferred after five full days of progesterone. For day-6 blastocysts there is still no firm standard for the best timing.

Should progesterone be measured before the transfer, and what level is enough?

In a programmed HRT cycle I find progesterone measurement useful: there is no corpus luteum, so the blood level shows how well the progesterone given is being absorbed. But there is no single “good value” for everyone; the route, the dose, the timing of the blood test and the laboratory all affect the result. Especially in cycles using vaginal progesterone alone, levels below roughly 9–11 ng/mL have been linked to lower success (Labarta et al., 2021; Melo et al., 2021). If the level is low, additional subcutaneous or intramuscular progesterone can be considered. I do not use it as an absolute cancellation threshold.

How long do I continue progesterone and estrogen after a positive test?

In a programmed HRT cycle there is no corpus luteum, so I do not stop the medication as soon as the test is positive. Practice varies between centres, but estrogen and progesterone are usually continued until the placenta produces enough hormones, most often until weeks 10–12 of pregnancy. Some studies support reducing them earlier, but there is no single, universal week for stopping; the plan should follow the centre’s protocol.

Does aspirin thicken the lining? Should I take it before a frozen embryo transfer?

No. Aspirin has not been shown to reliably thicken the endometrium or to improve routine FET success, so I do not give routine aspirin as a treatment for a thin lining. In randomized studies, aspirin did not clearly increase endometrial thickness (Weckstein et al., 1997; Hsieh et al., 2000); in one randomized trial in frozen embryo transfer cycles, the lining was even thinner in the aspirin group (Davar et al., 2020). A Cochrane review found no difference in live birth with aspirin in IVF (Siristatidis et al., 2016). Aspirin is used only when there is a separate medical reason, such as thrombosis or antiphospholipid syndrome, or for pre-eclampsia prevention in pregnancy.

Sources

• Wei D, Qin Y, Sun Y, et al. Natural ovulation versus programmed regimens before frozen embryo transfer in ovulatory women: multicentre, randomised clinical trial. BMJ 2026;392:e087045.
• Roelens C, Racca A, Mackens S, et al. Artificially prepared vitrified-warmed embryo transfer cycles are associated with an increased risk of pre-eclampsia. Reprod Biomed Online 2022;44(5):915–922.
• Erden M, Mumusoglu S, Uyanik E, et al. Revisiting natural cycle frozen embryo transfer: a systematic review and meta-analysis. Hum Reprod Update 2026;32(4):439–457.
• Saupstad M, Colombo C, Bergenheim SJ, et al. Optimizing the protocol for modified natural cycle frozen embryo transfer (mNC-FET): a multicentre, single-blinded randomized controlled trial. Hum Reprod Open 2026;2026(1):hoag003.
• Martins WP, Nastri CO. Endometrial preparation with vs without ovulation for frozen-embryo transfer: systematic review and meta-analysis of randomized controlled trials. Ultrasound Obstet Gynecol 2026.
• Venetis C, Kiose K, Apostolidou M, et al. Modified natural vs. natural cycle for endometrial preparation in frozen embryo transfer: a meta-analysis. Fertil Steril 2026;125(5):833–844.
• Singh B, Reschke L, Segars J, Baker VL. Frozen-thawed embryo transfer: the potential importance of the corpus luteum in preventing obstetrical complications. Fertil Steril 2020;113(2):252–257.
• Labarta E, Mariani G, Paolelli S, et al. Impact of low serum progesterone levels on the day of embryo transfer on pregnancy outcome: a prospective cohort study in artificial cycles with vaginal progesterone. Hum Reprod 2021;36(3):683–692.
• Melo P, Chung Y, Pickering O, et al. Serum luteal phase progesterone in women undergoing frozen embryo transfer in assisted conception: a systematic review and meta-analysis. Fertil Steril 2021;116(6):1534–1556.
• Siristatidis CS, Basios G, Pergialiotis V, Vogiatzi P. Aspirin for in vitro fertilisation. Cochrane Database Syst Rev 2016;11:CD004832.
• Weckstein LN, Jacobson A, Galen D, et al. Low-dose aspirin for oocyte donation recipients with a thin endometrium: prospective, randomized study. Fertil Steril 1997;68(5):927–930.
• Hsieh YY, Tsai HD, Chang CC, et al. Low-dose aspirin for infertile women with thin endometrium receiving intrauterine insemination: a prospective, randomized study. J Assist Reprod Genet 2000;17(3):174–177.
• Davar R, Pourmasumi S, Mohammadi B, Lahijani MM. The effect of low-dose aspirin on the pregnancy rate in frozen-thawed embryo transfer cycles: a randomized clinical trial. Int J Reprod Biomed 2020;18(9):693–700.
• ESHRE Add-ons Working Group, Lundin K, Bentzen JG, et al. Good practice recommendations on add-ons in reproductive medicine. Hum Reprod 2023;38(11):2062–2104.
• Andreeva P, Oprova I. Highlights from ESHRE 2025: advances in endometrial preparation strategies for frozen embryo transfer. EMJ congress review, 2025.

Dr. Senai Aksoy

Dr. Senai Aksoy studied and trained in France before returning to Turkey, where he was a member of the ICSI team at Sevgi Hospital, Ankara — the country's first ICSI centre (1994-95) — and a co-author on the first Turkish ICSI publications produced in collaboration with the Brussels Van Steirteghem group (Human Reproduction, 1996; PMID 8671323). He helped build the IVF programme at the American Hospital Istanbul and has been running his own fertility practice since 1998.

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The content has been created by Dr. Senai Aksoy and medically approved.