How Does Embryo Transfer work?

By (embryologist) and (babygest staff).
Last Update: 09/26/2019

Surrogacy is a reproductive process by which a woman, who is not going to be the mother, carries the pregnancy to term for another person or couple. In order to achieve pregnancy, in vitro fertilization (IVF) is performed, either with eggs and semen from the future parents or from a donor. In any case, once the embryos are obtained, they are cultivated and finally transferred to the uterus of the surrogate mother. That way, they can implant to give rise to a pregnancy.

When making the transfer, different aspects must be taken into account, including when to do it, how many embryos to transfer and what type of preparation to make the endometrium in an optimal state to receive the embryos.

Should the transfer be done at the 3rd or 5th day?

There are babies born of transfers on the 2nd day, 4th or even 6th day, but the most common thing is to perform the transfer on the third or fifth day of their development. The main reason is that it makes it possible to evaluate its quality according to its evolution up to that moment.

On the 2nd day it is still too early to know if the evolution is adequate, there is still much information missing. On the other hand, on the 4th day the embryo is in morula stage and there is no reliable standardized assessment of this stage of development, so it is preferable to wait until it is a blastocyst (day 5) to obtain more information about it.

If the transfer is made on the 3rd day, it means that it is 3 days since the egg was fertilized with a sperm, so the embryo is at least 72 hours old. In this cell stage, the embryo has 6-8 cells. An indication of good embryonic quality is when all cells are of the same size and have a homogeneous appearance.

Performing the transfer on the 5th day, after five days of culture, the embryos have reached blastocyst stage. At this stage, the embryo acquires a characteristic morphology: it is made up of more than 100 cells and two different cell types are differentiated, the trofoectoderm cells and the internal cell mass. In order to determine the embryonic quality, the appearance of both cell types is observed, as well as the degree of expansion of the blastocyst.

Depending on the number and quality of the embryos, it is decided whether to transfer on day 3 or day 5, this decision is made by the medical team. Each embryo has a different growth rate and therefore a different divison rate, so it is very common that in the same cycle there are more "advanced" embryos and other "slower". The fundamental aspects to select the best embryo for the transfer are the cell division rhythm and the embryonic morphology.

Number of embryos to be transferred

Before starting IVF treatment, the Surrogate mother and the intended parents should discuss embryo transfer and what their preferences are.

  • Parents (or father/mother in the case of single people) should consider whether they are willing to have twins, this possibility is greater depending on the number of embryos that are transferred. If two embryos are transferred, the probability of achieving pregnancy is greater, but also the risk of having two babies instead of one.
  • The surrogate mother must decide if she is willing to have more than one embryo transferred, knowing all the risks involved in a twin pregnancy and multiple birth.

Taking into account the agreement reached by both parties and the state of the embryos, the laboratory staff selects the embryo with the greatest possibility of implantation, or the best embryos in the case of accepting the transfer of several.

Frozen or fresh embryos?

After fertilization and development of the embryos in the laboratory, the most common is to transfer them fresh, that is, in the same cycle in which they are created, without going through a freezing and thawing process. However, there are certain situations in which it may be necessary to make a deferred transfer, i.e., of embryos frozen in another cycle.

A transfer of frozen embryos may be necessary when:

  • The transfer should be postponed in cycles in which a suitable endometrium for fresh transfer has not been developed.
  • A PGD is needed: if the biopsy is done on day 3, the embryos can be left in culture until day 5 without freezing them. However, if it is done on day 5, it would be necessary to freeze them until the results are obtained and to be able to decide which one or which ones to transfer.
  • Donated embryos from an embryo bank are used.
  • Frozen embryos from a previous IVF cycle are available.

The laboratory staff will advise the best option and will try to use whenever possible the embryos in fresh, since better results are obtained than with frozen ones.

Medication and transfer process

Before the transfer, it is very important to prepare the woman's endometrium to ensure that it is in optimal condition to receive the embryos and allow their implantation. For this purpose, hormonal medication is usually administered, although it can also be done in a natural cycle. The surrogate mother receives three types of medication at three different times:

  • First phase with oral contraceptives or GnRH antagonists to stop natural hormonal release from the ovary.
  • Second phase with estrogens to stimulate the growth of the endometrium and thus acquire the appropriate thickness and structure.
  • Third phase with progesterone to finish the endometrial preparation and favor the development of the pregnancy in its first stage.

Find all the information you need about pre-transfer medication here: Preparation of the endometrium for the transfer.

Once the endometrium is ready, the embryo is transferred. It is a simple process by which the embryos are deposited inside the uterus of the surrogate mother using an ultrasound machine. The whole process takes a few minutes and consists of the following steps:

  • A speculum is placed in the vagina. The vagina is cleaned with saline solution.
  • The cervix is cleaned and the cervical mucus is aspirated so that it does not interfere with the transfer.
  • The embryos are aspirated through the catheter (in the laboratory).
  • The catheter is inserted through the vagina and into the uterus.
  • The embryos are gently deposited in the uterine fundus.
  • The catheter is slowly removed.

The embryo transfer is completely painless, does not require anesthesia or any type of sedation. After placing the embryos, the woman must rest for about 20 minutes in the fertility centre itself. Afterwards she can go about her everyday life.

FAQs from users

Does the timing of the transfer change if PGD is done or if donated embryos are used?

By Sara Salgado (embryologist).

It will depend on when the PGD biopsy is done or when the donated embryos are in development. If the PGD biopsy is done on the 3rd day, the embryos can be transferred fresh on the 5th day or frozen for a subsequent transfer, either on the 3rd day, immediately after thawing, or on the 5th day, leaving them in culture for two days after thawing. However, if the biopsy is done on day 5, the embryos must necessarily be frozen.

In the case of donated embryos, it will also depend on the protocol to be followed by the centre. The transfer can be made as soon as they are thawed or you can wait to see how they develop.

Why isn't the transfer usually done on the 2nd or 4th day when the embryo is a morula?

By Sara Salgado (embryologist).

Because there is not enough information available to give an adequate assessment of the embryo quality and therefore to choose the best embryo. On the 2nd day, the embryo only has 4 cells, so it does not provide sufficient information on the rates of division or embryonic morphology, as it may happen that initially the embryos look very good but alterations appear later.

On the other hand, on the 4th day there is sufficient information about their development until the 3rd day, but as it has only begun to compact (forming the morula) it is not known whether it will give rise to a suitable blastocyst, which can be assessed on the 5th day.

Therefore, it is more appropriate to monitor embryonic development until day 3, when the embryo has 8 cells, or until day 5, when the embryo is already in the blastocyst stage.

When are embryos implanted after the transfer?

By Sara Salgado (embryologist).

In order to implant the embryo, it must form a blastocyst, expand and leave the zona pellucida (the outer layer that surrounds it), so implantation does not begin until 6-7 days after fertilization. If the transfer is made on day 3, it would take 2-3 more days for the time to begin implantation to arrive, and if it is made on day 5, 1 or 2 more days.

Can rest influence whether the transfer is positive or negative?

By Sara Salgado (embryologist).

A 20-30 minute rest period in the reproduction clinic is sufficient. The patient can return to a normal life, does not need any more rest and does not need to suspend her usual daily activities. It is important to bear in mind that in natural pregnancies the woman does not rest, since she does not know that fertilization has occurred and that the embryos are trying to implant in the endometrium.

Suggested for you

As we have seen, when deciding how many embryos to transfer or on what day it is best to perform the transfer, embryo quality is a key factor. In the following article you can find more information on how embryos are classified according to their quality and treatment relevance: Embryo quality In order to achieve good embryo quality, one of the most relevant aspects is that in vitro culture conditions are optimal to allow good embryo development. If you want to know more about this topic, we recommend you read the following post: In vitro embryo culture and development.

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 Sara Salgado
Sara Salgado
Degree in Biochemistry and Molecular Biology from the University of the Basque Country (UPV/EHU). Master's Degree in Human Assisted Reproduction from the Complutense University of Madrid (UCM). Certificate of University Expert in Genetic Diagnosis Techniques from the University of Valencia (UV). More information about Sara Salgado
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

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