Frozen Embryo Transfer (FET) in IVF

By (embryologist) and (babygest staff).
Last Update: 10/10/2019

In an in vitro fertilization (IVF) cycle, either for ones own cicle for a surrogacy, the remaining embryos are frozen for later use if the first transfer does not result in pregnancy or if the parents want to have a sibling after the birth of their first child.

This results in the purported deferred transfer or transfer of frozen embryos, which is nothing more than the use of cryopreserved embryos after thawing.

Embryo Freezing

The normal number of embryos to be transferred is usually between one and three. In this way, we improve the probability of pregnancy without increasing the risk of multiple gestation (twins, triplets...). However, on many occasions, the viable embryos obtained after the IVF process are more than those to be transferred. This results in remaining embryos, which will be cryopreserved in liquid nitrogen at -196 °C.

The high survival rates of the new cryopreservation methods allow these embryos to be used in subsequent cycles for a second pregnancy.

Thus, if after transferring the embryos to the uterus of the pregnant woman gestation is not achieved, a new attempt can be made without the need to repeat the ovarian puncture and fertilization, provided that there are viable embryos left over from the previous cycle and they have been frozen.

The current method of embryo freezing is embryo vitrification. This process allows the viability and quality of the embryo to be maintained after thawing or devitrification thanks to the ultra-fast temperature drop, which prevents the formation of ice crystals inside the embryo's cell. Vitrification is a cryopreservation system that keeps the embryos in practically the same state in which they were frozen. As a result, the success rates of frozen embryo transfer are high.

Transfer of frozen embryos step by step

The protocol followed in cases of frozen embryo transfer is simpler than when full IVF is performed, as ovarian stimulation, follicular puncture and fertilization are not necessary.

We skip all these steps and go directly to the endometrial preparation of the woman who is going to become pregnant, known in surrogacy as a gestational carrier. There are also those who call her surrogate mother but it is important to insist that this qualifier is not correct because the pregnant woman will not be the mother of the future baby, but only the woman who will gestate it.

The steps followed in both methods are the following:

Endometrium preparation

Endometrial preparation consists of administering hormonal medication to the gestational carrier so that her uterus is receptive and the embryo is able to implant into it, thus allowing pregnancy.

The first step is to give estrogen, either orally or in patches. Normally, medication is administered between the first and third day of the cycle.

For a better control of the cycle, it is possible to give oral contraceptives or GnRH antagonists in the cycle prior to the transfer of frozen embryos.

This treatment is usually continued for 10 to 13 days. At this time, the specialist must monitor the evolution of the endometrium (uterine layer) via ultrasound to determine when is the right time for transfer.

The receptive endometrium is characterized by a trilaminar aspect (three parallel lines are observed in the ultrasound) and a thickness of between 7 and 10 mm.

In addition to estrogen, a woman receives progesterone orally or vaginally. It is generally recommended to start taking progesterone three or four days before the transfer, to improve the conditions of the uterus and promote implantation in the endometrium. Therefore, at the time the transfer is scheduled, the protocol to be followed regarding the administration of progesterone is indicated.

Thawing of embryos

Once the ideal time for the transfer has been programmed, the embryos are thawed. They will be defrosted one or the other day depending on the day of development in which they were frozen and the stadium in which they are to be transferred.

Thus, if the embryos were frozen on day 3 and are to be transferred at this stage, they will be thawed on the same day as the transfer. If, on the other hand, they are to be transferred in blastocyst stage (day 5), they will be thawed two days before the transfer and left in culture until then.

For thawing (or devitrification), simply remove the embryos from the bank, where they are at -196°C, and quickly introduce them into a special medium at 37°C. They must then pass through a number of specific media until they regain their pre-freezing state.

It is essential that the transition from one temperature to another is made as quickly as possible.

Embryo transfer

The transfer consists of depositing the embryos in the uterus of the gestational carrier. For this, the embryos are aspirated with a special cannula and this is carefully introduced into the uterine cavity through the vagina. Once there, the contents of the cannula, which includes the embryos, are released.

This procedure is simple:

  • It can be performed either in the operating room or in the gynecologist's own practice.
  • No anesthesia required.
  • It's fast: it only takes about 15-20 minutes.
  • It is performed via ultrasound, that is, visualizing the uterus by ultrasound. This facilitates the deposition of embryos and avoids tearing or altering the uterine cavity.
It is recommended that the woman who is going to receive the embryos go to the transfer with the bladder full to facilitate the vision of the uterus by the gynecologist.

It is not always easy to decide whether to make the transfer on day 3 or day 5-6 of embryonic development. To make this decision, numerous factors must be taken into account, such as the quality of the embryos before and after freezing, the number of embryos available, the background of each situation, etc. In this article you’ll find more information about the best moment for embryotransfer: When to do the embryo transfer?

Another important decision regarding the transfer is the number of embryos to be transferred, as the transfer of two or more embryos considerably increases the likelihood of multiple pregnancy. Some of the aspects that are considered in deciding how many embryos to transfer can be found in this article: How many embryos to transfer?

Transfer in Surrogacy

In a surrogacy process, the gestational carrier is the woman who undergoes endometrial preparation and embryo transfer.

In this case, whether fresh or frozen embryos, the treatment followed by the gestational carrier is exactly the same.

Generally, at least the very first transfer will be attempted in fresh. For this purpose, it is necessary to synchronize the cycles of the surrogate and the woman who provides the eggs (preferably the future mother or, in case of disability, a donor). When this is not possible, frozen embryo transfer is used.

Also, if the first transfer does not result in pregnancy, there will be successive transfers of the remaining embryos that were cryopreserved until trying for pregnancy.

Finally, if, after the birth of the first child, you wish to have a second child, you can use the remaining embryos that are frozen. Thus, this second process of surrogacy will be simpler. It can be done with the same gestational carrier or, on the contrary, with the help of a different woman.

FAQs from users

How much rest time is needed after frozen embryo transfer?

By Andrea Rodrigo (embryologist).

Whether the transfer is of fresh embryos or after freezing and thawing of the embryos, only a resting time of about 20-30 minutes is recommended. After this time, the woman can return home on her own foot and continue with her daily routines, except in special cases in which the doctor indicates otherwise.

What does it mean to transfer frozen embryos in a natural cycle?

By Andrea Rodrigo (embryologist).

The natural cycle embryo transfer consists of transferring the embryos without the previous treatment of endometrial preparation. To do this, the woman's natural cycle is simply controlled and the endogenous release of estrogens and progesterone is taken advantage of.

The problem with this process is that natural hormones are not always sufficient to favor implantation and pregnancy. This will depend on each woman and each case. In surrogacy, natural cycle treatments are not usually done.

How is the endometrium measured to calculate when frozen embryos will be transferred?

By Andrea Rodrigo (embryologist).

The measurement of the endometrium is performed by the gynecologist using transvaginal ultrasound. This tool allows the uterus to be analyzed without causing harm to the woman.

Suggested for you

We have talked about the embryo transfer process when embryos go through a previous vitrification-devitrification process, but the transfer process itself is the same as when embryos are fresh. Do you want to know the details? More about here: What is embryo transfer?

We have also talked about the treatment that the gestational carrier goes through to prepare her endometrium before the transfer. In this article we explain in detail the protocol followed: Endometrium preparation

Finally, We recommend you consult this article if you want to know more about the process of freezing and thawing embryos before their transfer: What is embryo vitrification?

We make a great effort to provide you with the highest quality information.

🙏 Please share this article if you liked it. 💜💜 You help us continue!



 Andrea Rodrigo
Andrea Rodrigo
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia. Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia along with the Valencian Infertility Institute (IVI). Postgraduate course in Medical Genetics. More information about Andrea Rodrigo
Adapted into english by:
 Romina Packan
Romina Packan
Babygest Staff
Editor and translator for the Babygest magazine in English and German. More information about Romina Packan

Find the latest news on assisted reproduction in our channels.