Frozen Embryo Transfer (FET)

Reviewed by Dr. Priyadatt Patel, MBBS, MS (Obstetrics & Gynaecology) — Senior Gynecologist · Advanced Laparoscopic Surgeon · IVF and Endometriosis Programme Lead, Advanced Laparoscopic Surgeon, IVF and Endometriosis Programme Lead, Balaji Horizon Women’s Hospital, Science City Road, Ahmedabad. Last reviewed: 5 August 2026.

Frozen embryo transfer (FET) places a thawed, previously frozen embryo into the uterus in a later, specially prepared cycle. For most women it gives live-birth rates similar to a fresh transfer; for those at risk of ovarian hyperstimulation, or with PCOS, freezing first is often both safer and more successful. The right choice depends on your cycle, your health and your embryos.

A decade ago, a frozen embryo transfer was mostly a second chance — what you did with the embryos left over after a fresh transfer had not worked. Today it is often the first plan, and sometimes the safer one. That shift is one of the quieter but more important changes in modern fertility care, and it is worth understanding properly, because “fresh or frozen” is not a question of which is newer or better in the abstract. It is a question of what suits your cycle, your health, and your embryos.

This page explains what a frozen embryo transfer (FET) is, why freezing has become routine rather than exceptional, what the evidence actually shows about outcomes and safety, and — just as importantly — when a freeze-all approach is not needed. It is written for general orientation, not as a substitute for a plan made with your own fertility team.

In IVF, eggs are collected and fertilised in the laboratory, and the resulting embryos are grown for a few days. In a fresh transfer, an embryo is placed into the uterus within that same stimulation cycle. In a frozen embryo transfer, the embryos are instead cryopreserved — frozen and stored — and one is thawed and transferred in a later, separate cycle.

The distinction matters because the two approaches ask the uterus to receive an embryo under very different conditions. A fresh transfer happens in a cycle where the ovaries have been stimulated with high doses of hormones, which changes the hormonal environment of the uterine lining. A frozen transfer happens later, in a calmer cycle prepared specifically to receive an embryo. Everything else in this article follows from that single difference.

Why freezing became routine, not exceptional

Three developments turned freezing from a compromise into a mainstream option. The first is technical: modern cryopreservation uses vitrification, an ultra-rapid freezing method that avoids the ice-crystal damage of older slow-freezing techniques. Systematic reviews informing World Health Organization guidance have established vitrification as the more effective method, and embryo survival after thawing is now very high in good laboratories (Hum Reprod Update 2017). A frozen embryo today is, in most cases, not a compromised embryo.

The second is the move toward single-embryo transfer to avoid the real risks of twin and triplet pregnancy. Freezing lets a clinic transfer one embryo at a time and keep the rest safely in storage for future attempts, rather than transferring several at once. If you would like to understand that trade-off in depth, see our page on deciding how many embryos to transfer.

The third is the freeze-all strategy — deliberately freezing every suitable embryo and transferring none fresh — which allows the transfer to be separated from the stimulation cycle entirely. Whether that separation actually improves anything is the question the evidence has spent the last decade answering.

It is tempting to assume that a calmer, purpose-prepared cycle must produce more pregnancies. For most women, the large trials do not support that assumption.

In ovulatory women having their first IVF cycle, a multicentre randomised trial found no meaningful difference in live birth between freezing all embryos and transferring fresh (48.7% with frozen versus 50.2% with fresh; N Engl J Med 2018). A separate randomised trial of single-blastocyst transfer in ovulatory women in China explored the same question with extended culture (Lancet 2019). The 2021 Cochrane review of fresh versus freeze-all concluded that, for the general IVF population, the freeze-all strategy does not clearly increase cumulative live birth — while it does change the balance of certain risks, discussed below.

The honest summary is this: for most women, freezing all embryos does not, by itself, make a pregnancy more likely. A freeze-all cycle is not an “upgrade,” and it should never be sold as one.

Who genuinely benefits from a freeze-all approach

There are, however, clear situations where deferring the transfer is the better and sometimes safer choice:

• High risk of ovarian hyperstimulation syndrome (OHSS), particularly in women with polycystic ovary syndrome (PCOS) or a very strong response to stimulation. Freezing all embryos and transferring later avoids a pregnancy hormone (hCG) compounding OHSS, and in women with PCOS, trials have shown a freeze-all approach can both reduce OHSS and improve live-birth rates. This is the single strongest, best-evidenced indication.
• Preimplantation genetic testing (PGT), where embryos are biopsied and must be frozen while results are awaited.
• A uterine lining that is not ready in the fresh cycle — for example, a thin or out-of-phase endometrium, fluid in the cavity, or a progesterone rise before egg collection.
• A medical or practical reason to wait — a polyp or fibroid to be treated first, a condition to stabilise, or simply the need to separate egg collection from transfer.
• Fertility preservation, where embryos or eggs are frozen deliberately for use months or years later. Our overview of fertility preservation and egg freezing covers this in detail.

In each of these, freezing is not a reflex — it is a decision matched to a specific finding.

The corollary matters just as much. If you are an ovulatory woman with a normal response to stimulation and a receptive uterine lining, the evidence does not say you must freeze everything. A blanket “we freeze all embryos for everyone” policy applies a strategy designed for higher-risk situations to women who do not need it, adding a cycle of waiting — and, as the next sections explain, a different set of obstetric considerations — without a clear benefit in return. The right question is never “fresh or frozen?” in the abstract, but “is there a specific reason, in this cycle, to defer the transfer?”

Preparing the uterus: natural versus medicated cycle

Once a frozen transfer is planned, the uterine lining has to be prepared to receive the embryo, and there are two broad ways to do it.

In a natural (or modified natural) cycle, the transfer is timed to your own ovulation, so your body’s own corpus luteum — the structure left behind after an egg is released — produces the progesterone that supports early pregnancy. In an artificial (also called programmed or medicated) cycle, ovulation is suppressed and the lining is built entirely with oestrogen and progesterone medication. There is no corpus luteum in a programmed cycle.

For a long time this was treated purely as a matter of convenience and scheduling, because pregnancy rates are broadly similar between the two. Over the last few years it has become clear that the choice also has a safety dimension — and that is a genuinely important, and still-evolving, part of modern FET care.

The safety conversation: blood pressure and the corpus luteum

A consistent signal has emerged from the research: pregnancies conceived through frozen transfer in an artificial (programmed) cycle carry a somewhat higher risk of hypertensive disorders of pregnancy, including pre-eclampsia, compared with frozen transfers in a natural cycle or with fresh transfers (reviewed in J Gynecol Obstet Hum Reprod 2021; retrospective cohort in Int J Gynaecol Obstet 2024).

The likely explanation is the missing corpus luteum. Beyond progesterone, the corpus luteum releases hormones such as relaxin that help the mother’s cardiovascular system adapt to pregnancy — widening blood vessels and increasing the flexibility of the arteries. Research has shown that programmed cycles, which have no corpus luteum, are associated with a blunted version of these normal early-pregnancy cardiovascular changes (Am J Obstet Gynecol 2021). A 2023 systematic review and meta-analysis concluded that natural-cycle FET is associated with a lower risk of adverse obstetric and neonatal outcomes than artificial-cycle FET — reframing endometrial preparation as a question of safety, not only of effectiveness (Hum Reprod Update 2023).

This does not mean artificial cycles are unsafe or should never be used; they remain necessary for many women, for example those with irregular or absent ovulation. It means the choice deserves a real conversation. Where a natural or modified natural cycle is feasible, there is now a reasonable safety argument for preferring it — and where a programmed cycle is used, it is sensible to be attentive to blood pressure in pregnancy. This is an area where good practice is still being refined, and where a thoughtful clinic individualises the decision rather than defaulting to whichever cycle is easiest to schedule.

Birthweight and other neonatal considerations

The fresh-versus-frozen difference shows up in the baby’s growth as well, in a way researchers are still working to fully explain. On average, babies born after frozen transfer tend to be heavier, with a higher chance of being large for gestational age, while babies born after fresh transfer lean the other way, toward lower birthweight and small-for-gestational-age. Neither pattern is a reason for alarm, and both fall within the range of routine antenatal monitoring, but they are part of why the choice of transfer type is a clinical decision rather than a purely logistical one. Careful pregnancy care — including well-timed growth scans through our fetal medicine and pregnancy scanning service — is the appropriate response, whichever route led to the pregnancy.