What is the difference between fresh and frozen embryo transfer?

The key difference between a frozen and fresh embryo transfer cycle is whether or not the embryos have been previously cryopreserved, versus if they are from a current IVF cycle and thus have never been frozen. In a fresh embryo transfer, the embryo(s) is/are transferred during a cycle where the ovaries were hyperstimulated, which is not the case with frozen transfers. A fresh embryo transfer usually occurs 3 or 5 days after egg retrieval and fertilization, with the day of fertilization referred to as “Day 0”.  A day 3 embryo, also called a cleavage stage embryo, has approximately six to ten cells in total. A day 5 embryo has approximately 150-200 cells and is called a blastocyst.^iv The American Society of Reproductive Medicine (ASRM) has published guidelines on embryo transfer. Their recommendation is to transfer one embryo (known as elective single embryo transfer) in most cases to minimize twin pregnancies, ^v and for patients with good prognosis to transfer blastocysts instead of cleavage-stage embryos.^vi The number of embryos transferred depends on patient age and embryo quality.

‍

Because frozen embryos are cryopreserved, they must be thawed prior to being transferred. Overall, this process is now very safe. Advancements in cryopreservation and thaw techniques have increased the chance of embryos surviving the freezing and thawing process (thaw survival rates).^vii The major advancement in cryopreservation has been utilization of a technique called vitrification. Vitrification involves freezing the embryos at a rapid rate using cryoprotecting agents to reduce the amount of water in the cell, which minimizes the formation of ice crystals that could damage the embryo.^viii On average, 95 to 98 percent of frozen embryos survive the thaw after vitrification.^ix,x The previously used technique of slow-freezing embryos had much lower embryo survival rates.  

‍

The pregnancy rates after thawed embryo transfer are similar to those with fresh embryo transfer, as discussed in detail below. The freezing process tends to harden the outer shell of the embryo (zona pellucida) so thawed embryos are usually put through a procedure called “assisted hatching;” this involves using a laser to create a hole in the zona pellucida to help the embryo hatch out of its shell and better implant in the uterus at the time of transfer. However, laser-assisted hatching is optional for fresh embryo transfers. According to the latest ASRM guidelines (2022), it does not appear that laser assisted hatching improves live birth rates in unselected population (i.e.  not studies that selected patients with diminished ovarian reserve, advanced maternal age, etc.). Furthermore, the published evidence has mixed results as to whether laser assisted hatching improves live birth rates in poor prognosis patients and in frozen transfers.^xi  

‍

Assisted hatching is also used when performing embryo biopsy for preimplantation genetic testing (PGT). Hatching allows the embryos to come out of the zona to allow for biopsy of the trophectoderm (cells destined to be the placenta) and not the inner cell mass (cells destined to become the embryo).

‍

The frozen embryo is usually thawed and prepared for transfer (post-thaw culture) on the day of the procedure, a few hours before a patient arrives for the transfer appointment. Embryos that have been frozen at the cleavage stage may undergo an additional culture to reach blastocyst development before being transferred. However, most clinics now prefer to freeze embryos at the blastocyst stage and thaw them on the day of embryo transfer, usually in the morning.^xii Following the thaw, the clinic will evaluate whether the embryo survived the thaw and may re-grade the embryo. The embryologist will look at whether the embryo is still intact, referring to the percentage of cells that are still viable, and ensure there are no signs of injury to the zona pellucida post-thaw. When less than 50 percent of the embryo’s cells survive, the embryo is usually deemed unviable and not suitable for transfer - in other words, it did not survive the thaw.^xiii  

‍

Fresh and frozen embryo transfers usually require the patient to take additional progesterone and often supplemental estrogen (depending on the type of transfer).^xiv In addition, some clinics may perform a “mock embryo transfer” in advance of embryo transfer to help ensure the procedure will run smoothly.^xv Mock embryo transfers involve using an empty catheter (no embryo) to measure the path through the cervix and to the top of the uterus, to mimic what would occur during the actual transfer. This is often done in the clinic.

‍

The transfer procedure itself is identical for fresh and frozen embryo transfers. In both cases, the patient is lying on their back with legs raised (knees bent and calves supported by padded leg supports). A speculum is inserted into the vagina and a soft transfer catheter that contains the embryo(s) is passed through the cervix and into the upper portion of the uterus. The embryo(s) is/are then gently injected from the catheter into the uterus under ultrasound guidance. The ultrasound may be on the abdomen, requiring a full bladder, or in the vagina, requiring an empty bladder. There is usually no anesthesia given, and the patient goes home the same day ^xvi Patients can often watch the transfer process on an ultrasound monitor or TV screen. Embryo transfer is usually well-tolerated and does not require any analgesia or anesthesia in most cases. ^{xvii xviii}

Why choose fresh embryo transfer?

There are various reasons that a patient may prefer a fresh embryo transfer; these include costs, travel time to the clinic, and fear of losing the embryo due to cryopreservation. Reasons related to success rates are discussed in a later section.  

A patient may also choose a fresh embryo transfer if they have only one or a few viable/good quality embryos after going through the egg retrieval and IVF process. A fresh embryo transfer would therefore minimize the potential for loss of any embryos through cryopreservation and thawing.^xix

The cryopreservation process mainly involves cooling the cells of the embryo down to sub-zero temperatures to stop all biological activity and save the embryo for a future transfer. Despite advancements in the cryopreservation technique, concerns remain regarding the impact of the freeze-thaw cycle on embryo quality and the potential to lose embryos during the process. Currently, there are several protocols that exist for the freezing and thawing process involving different cryoprotectants (chemical agents that protect cells against the formation of ice crystals and cell death due to freezing). This lack of standardization can lead to inconsistency between frozen embryo transfer survival rates across clinics.^xx It is important to discuss anticipated embryo thaw survival rates with one’s doctor.

Clinicians and embryologists often will not freeze low quality embryos, such as grade CC blastocysts, due to reduced chance of thaw survival and/or implantation.^xxi As such, some patients may choose to transfer them fresh instead of discarding them, especially given that low-grade embryos can still be euploid (genetically normal) and lead to healthy live births. One study showed that although the live birth rate of “C” grade embryos was lower than that of “A” or “B” grades, transfers of CC embryos still resulted in an overall live birth rate of 13.3 percent.^xxii Although this was one-third the live birth rate of AA embryos, it highlights that many CC graded embryos lead to live birth.^xxiii Another study had similar results, with live birth rates of 17 percent from CC grade embryos.^xxiv The authors of this article suggest that CC blastocysts should be transferred and not discarded, especially for infertile women who may only have poor-quality embryos.^xxv These findings may encourage those with low grade blastocysts to transfer them in a fresh transfer as opposed to discarding them, or risk losing them in the freeze-thaw process.  

Women may also decide on a fresh embryo transfer because of advanced maternal age. As more women are choosing to delay childbearing, there is a time constraint involved with delaying an embryo transfer to a subsequent cycle after egg retrieval and fertilization. Patients may not want to delay their age at delivery any further. In addition, embryos from women above 40 years old may be more fragile (because they were created from older oocytes) and some women may be worried about reduced quality or losing embryos during the freeze-thaw process. One study by Chen et al compared pregnancy outcomes in 720 women of advanced maternal age (375 receiving a first fresh embryo transfer vs. 345 receiving a first frozen embryo transfer).^xxvi The clinical pregnancy rate was lower in the frozen embryo transfer group (26.4 percent vs. 33.6 percent), a statistically significant result, though there was no difference in live birth rate between fresh and frozen transfer.^xxvii

For some women, cost might be another decisive reason to choose fresh transfer. Fresh embryo transfers usually cost less because the fresh transfer is included in an IVF treatment package, whereas frozen embryo transfers have additional costs. These extra costs can include the embryo cryopreservation, annual embryo storage fees (to keep the embryos frozen), the laser-assisted hatching procedure, and the frozen transfer itself. Because the transfer is electively done in a later cycle, this would also be associated with clinical costs to prepare for a frozen embryo transfer cycle, such as endometrial priming medications, cycle monitoring, embryo thawing, and transfer.^xxviii

The factor of cost and time can be especially relevant for those travelling out of town or overseas to receive IVF treatment. Receiving a fresh transfer would reduce the number of trips to their fertility clinic.  

Why choose frozen embryo transfer?

There are various reasons why a provider or woman may plan for a frozen embryo transfer. If a patient’s progesterone level rises or her estrogen levels are too high, frozen transfer is preferred. Additionally, providers may choose a “freeze all” approach to minimize the risk of ovarian hyperstimulation syndrome (OHSS) and when planning embryo genetic testing There are also elective reasons such as optimizing timing of a pregnancy or when banking embryos for family planning. Reasons related to success rates are discussed in a later section.

Necessary reasons for frozen embryo transfer

In some cases, patients must choose frozen embryo transfer(s) for clinical reasons, such as a higher risk of OHSS, potential asynchronization of the endometrium, and pre-implantation genetic testing, among others. This is often termed a “freeze-all” strategy, because all the IVF embryos are cryopreserved to be transferred in a later cycle (as frozen embryo transfers).^xxix

One advantage of transferring a frozen embryo later is reducing the risk of an overreaction to the fertility drugs, a condition known as ovarian hyperstimulation syndrome (OHSS), which can occur after a fresh embryo transfer.^xxx OHSS is a condition in which patients have fluid collect in their abdomen (ascites) and on their lungs (pleural effusion), electrolyte abnormalities, and an increased risk for blood clots. The risk of OHSS is significantly reduced with a freeze-all approach.

Another advantage of frozen embryo transfers is that the uterine environment may be more physiologically normal without the effects associated with ovarian stimulation medication (i.e., follicle-stimulating hormone and luteinizing hormone). This is because the medication usually leads to high estrogen levels, which can impact the endometrium (the part of the uterus in which the embryo implants). Studies have shown that controlled ovarian stimulation may potentially have a negative effect on endometrial growth and receptivity,^xxxi depending on hormone levels, egg counts, and patient history.

Women that typically have a thin endometrium around the time of transfer may benefit from frozen embryo transfer. Adequate growth and development of the uterine lining (the endometrium) is mainly affected by estrogen. Endometrial thickness has been associated with implantation rate. IVF medications impact estrogen levels and therefore have the potential to interfere with endometrial maturation and receptivity, and hence implantation rate.^xxxii  

Some studies have suggested that hormone replacement therapy in a frozen embryo transfer cycle may allow for better endometrial receptivity in patients with a thinner endometrium. For example, a study by Guo et al (2020) demonstrated that women with a thin endometrium had significantly higher live birth (31 percent vs. 18 percent), clinical pregnancy (40 percent vs. 26.4 percent), and biochemical pregnancy (46.2 percent vs. 32.9 percent) rates after receiving a frozen embryo transfer compared to the group that received a fresh embryo transfer.^xxxiii

When embryos need to be pushed to Day 6 or Day 7 to reach the blastocyst stage, these embryos will be frozen instead of transferred fresh, since evidence shows that Day 6 fresh blastocyst transfers have lower success rates compared to Day 5 fresh transfers.^xxxiv This is believed to be a result of the endometrial receptivity being out of sync with embryo development (since the embryos took one extra day to reach the blast stage, meaning one day past the optimal day for transfer). Embryo cryopreservation would enable better synchronization between embryonic and endometrial development compared to fresh embryo transfers in the case of Day 6 or Day 7 blastocysts.^xxxv

Finally, a frozen transfer is necessary when patients are doing preimplantation genetic testing (PGT), because patients need to wait for the results to come back before they can proceed with a transfer. Results are not usually available for 2-4 weeks, necessitating a frozen embryo transfer.

Elective reasons for frozen embryo transfer

Some patients may elect a frozen embryo transfer because of personal reasons, such as the desire to schedule a transfer on a date that is more convenient for them instead of 3 or 5 days after their egg retrieval.^xxxvi Others may choose to undergo several cycles of egg retrieval and store entire cohorts of their fertilized embryos until they are ready to get pregnant, or until they feel they have enough embryos “banked” to complete their family. This allows for more embryos to be created when the patient is younger, with better egg quality, compared to waiting until after a first baby (or after a series of failed transfers) to do a second egg retrieval.^{xxxvii,xxxviii} In this case, the frozen embryo transfer may happen years after embryo creation. This process of embryo banking obviously necessitates frozen embryo transfers and is generally more common in patients starting their fertility journey at a later age and/or with diminished ovarian reserved (DOR). This same approach applies to patients banking embryos due to upcoming medical treatments, such as cancer-related surgery, chemotherapy, or gender-affirming therapy.  

For those who undergo fresh embryo transfer and have extra embryos, they will cryopreserve those embryos for transfer at a later date if the first transfer was unsuccessful or when planning another pregnancy.^xxxix Finally, the process of controlled ovarian stimulation (also called hyperstimulation) is physically and emotionally challenging for some women, and they may feel that they want a break before transferring an embryo. As such, they choose to freeze their embryo(s) and wait until a later cycle after they feel they have had time to rest, both physically as well as mentally.

Success rates of fresh versus frozen embryo transfer

Patients often ask the question “Should I do a fresh or frozen embryo transfer?” because they want to optimize their chance of success. Several studies have attempted to compare reproductive and IVF outcomes between fresh and frozen embryo transfers. The results between studies are not always consistent, but the overall trend across published research shows that the clinical pregnancy and live birth rates are similar for fresh and frozen embryo transfers for patients with normal response in an IVF cycle. As such, each patient should discuss her specific case with her physician. The results from various studies and meta-analysis are described below.  

US IVF clinics report outcome data to the CDC, which compiles an Annual National ART Summary Report.^xl Historical trends favored fresh transfers. However, as cryopreservation techniques have improved, FETs have become more successful compared to transfers performed after previous slow freezing methods. Comparing recent data on fresh vs. frozen transfer using a patient’s own eggs would be misleading since FETs include transfers after preimplantation genetic testing, which would skew the results in favor of FET. However, a good comparison is examining donor egg fresh embryo transfers versus donor egg frozen embryo transfers. For example, from the 2015 annual report, the percentage of transfers resulting in live births was 55.6 percent for fresh transfers using donor eggs, versus 42.3 percent for frozen transfers using donor eggs.^xli Data on fresh vs. frozen transfer results with donor eggs was not reported in the annual summaries from 2016 onwards. In contrast, other studies show no differences in outcomes between fresh versus frozen transfers, as discussed below.  

A recent randomized controlled trial of over 600 women compared those randomly assigned to undergo a fresh transfer to those randomly assigned to receive a fresh transfer.^xlii Inclusion criteria included those with at least three good quality Day 3 embryos in couples undergoing their first, second, or third IVF/ICSI cycle (female age 18-42yr). Cycles with donor gametes, PGT, or planning for freeze-all cycles were excluded. The live birth rate was higher in fresh transfers (34.3 percent) compared to frozen embryo transfers (28.3 percent), but this was not statistically significant.^xliii

Similarly, a Cochrane meta-analysis using data from eight randomized control trials, which included a total of 4 712 women undergoing cleavage-stage and/or blastocyst transfer, reported no significant difference in cumulative birth rate between the freeze-all strategy (58 percent) and the fresh embryo transfer (57-63 percent). ^xliv The authors report there is little evidence to support that one strategy is better than the other in terms of live birth rate and ongoing pregnancy rate.^xlv

A randomized controlled trial (RCT) published by Shi et al assessed 2 157 ovulatory women who underwent their first IVF cycle with either a fresh or frozen embryo transfer (up to two cleavage-stage embryos were transferred per patient).^xlvi They reported no significant differences in the live birth, implantation, clinical pregnancy, overall pregnancy loss, and ongoing pregnancy rates between the two groups. As expected, those undergoing frozen transfer had a lower rate of ovarian hyperstimulation syndrome.^xlvii

When looking at annual summary statistics across European clinics, similar clinical pregnancy rates are reported for both fresh and frozen transfers. The European Society for Human Reproduction (ESHRE) collects assisted reproductive technology data generated by national registries, clinics, or professional societies. The 2018 report included 1 422 clinics across 39 countries and more than 1 million treatment cycles.^xlviii Traditionally, delivery rates were higher for fresh embryo transfers (1997 to 2016). However, in 2017 and 2018 delivery rates per transfer were similar between fresh and frozen embryo transfers using patients’ own eggs (approximately 25 percent).^xlix

Under certain conditions, studies have shown higher pregnancy rates with frozen embryo transfer. One of these situations is when progesterone levels elevate during an IVF cycle, purportedly shifting the window of implantation and thus decreasing pregnancy rates with fresh transfer. Two studies compared IVF outcomes of Day 3 fresh vs. frozen embryo transfer cycles where progesterone levels were elevated (>1.5 ng/mL on the day of trigger). Both studies observed significantly better IVF outcomes in the group using the freeze-all strategy compared with the group completing fresh embryo transfers.^l,li The effect of elevated progesterone on Day 5 fresh embryo transfers, which are now much more common than Day 3, has also been assessed. Some studies examining Day 5 fresh embryo transfers in those with elevated progesterone found no statistically significant difference between the fresh and freeze-all groups, whereas others found better outcomes for the freeze-all approach when serum progesterone was elevated.^{lii,liii,liv,lv} As such, providers may suggest FETs (instead of fresh ET) when progesterone levels are prematurely elevated in a cycle.

Another situation in which frozen embryo transfer is preferred over fresh transfer is in the setting of high numbers of eggs retrieved, which is usually associated with high estrogen levels. These supraphysiologic hormones levels are thought to be detrimental to endometrial development. For example, in a study of polycystic ovarian syndrome (PCOS) patients completing Day 3 embryo transfers, it was found that frozen embryo transfers were associated with a higher rate of live birth than fresh transfer cycles.^lvi A 2020 study found that patients with >11 oocytes retrieved had higher cumulative live birth rates using a freeze-all strategy of blastocysts compared to those who used fresh transfer (for patients <37yr undergoing blastocyst transfer).^lvii They did not observe the benefit of a freeze-all strategy in patients who retrieved less than 11 eggs.^lviii

A recent study also promoted FET over fresh transfer in blastocyst transfers. A 2019 RCT by Wei et al involving 1 650 women with regular menstrual cycles undergoing their first IVF cycle reported higher rates of singleton live births associated with a frozen single blastocyst transfer compared to a fresh single blastocyst transfer.^lix  

It is worth noting that patients often ask how long vitrified embryos can be stored. Whether or not a long storage duration worsens pregnancy rates and/or neonatal outcomes is unclear, and the evidence is inconsistent.^lx, ^lxi,^lxii,lxiii. There is some evidence to suggest that storage of frozen embryos for more than six years could decrease thaw survival rates and clinical pregnancy rates.^lxiv. On the other hand, other studies have not found a negative impact of storage length on outcomes,^{lxv, lxvi, lxvii} and more research is needed to clarify the impact of storage.

Obstetric and perinatal outcomes in fresh versus frozen embryo transfers

Another factor patients often consider involves differences in obstetric (pregnancy-related complications) and/or perinatal outcomes (preterm birth, stillbirth, or early infant loss within a week of delivery) for fresh vs. frozen transfers. Similar to findings for IVF pregnancy success rates (discussed above), results are also conflicting when considering obstetric and perinatal outcomes between fresh and frozen transfers.  

A systematic review and meta-analysis evaluating six clinical studies reported an increased risk of obstetric complications in pregnancies after frozen embryo transfer compared to fresh transfer. These complications included pregnancy-induced hypertension, pre-eclampsia, and placenta accreta.^lxviii Further, a 2020 Cochrane meta-analysis found that among 4 172 women undergoing IVF transfers, the freeze-all strategy may increase the risk of pregnancy hypertension and large‐for‐gestational‐age baby - although the authors cautioned that more research is required.^lxix  

In contrast, one recent study by Herman et al did not find evidence for differences in obstetric and perinatal outcomes (birthweight, small for gestational age, and preterm birth) between a single fresh vs. frozen embryo transfer in the same woman (a sibling comparison).^lxx They compared 107 fresh transfer pregnancies matched to 107 frozen transfer pregnancies with live singleton deliveries (>24 weeks of gestation) and concluded that siblings delivered after fresh and frozen embryo transfers had similar prognosis and development.^lxxi

In addition, the RCT by Wei et al (described above) also reported that the risks of developing OHSS, pregnancy loss, other obstetric complications, and neonatal morbidity were similar between the two groups, except for a higher risk of pre-eclampsia in the frozen blastocyst transfer group.^lxxii  In contrast, a study using SART data from 2004-2006 found that fresh transfers were associated with low birth weight compared to frozen transfers.^lxxiii  This data is older and based on Day 3 transfers, plus typically multiple embryos per transfer, and thus should be interpreted with caution.

Overall, given the potential for adverse obstetric outcomes after FET, some researchers suggest the freeze-all strategy should only be offered to certain patients with a clear clinical indication of benefit from this approach.^lxxiv On the other hand, other providers may offer the freeze-all strategy to all patients.  

Conclusion

There are many factors that go into decisions related to a fresh or frozen transfer, though many women may experience both options at different points in their IVF journey. Each patient should consult with her physician to determine the most appropriate transfer protocols. Given that evidence-based success rates are similar between fresh and frozen transfers, the right approach for one individual may be different for the next.