How Does Embryo Transfer work?

By Babygest Staff
Last Update: 02/23/2021

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 future parents or from a donor. In any case, once the embryos are obtained, they are kept in culture 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.

Definition of embryo transfer

Embryo transfer is the last stage of IVF. It is a simple procedure that consists of depositing the embryos in the uterus of the woman in charge of carrying the pregnancy to term.

Embryo transfer does not require anesthesia, since it is not a painful process for the woman. In addition, the embryos are usually placed in the uterus in an eco-guided manner.

Embryo transfer can be performed at any time during embryo development. However, it must be taken into account that the endometrium is receptive in order to favor implantation in the uterus.

The success of the embryo transfer will depend on the quality of the embryos, but also on the state of the endometrium and its receptivity at the time of the transfer.

Transfer on the 3rd or 5th day?

The time and day when the embryos are transferred will be advised by the specialists themselves according to the number of embryos, the quality, the condition of the endometrium, etc. 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 the 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 the 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 trophectoderm 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.

Related post: When to perform the embryo transfer?

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, but always considering medical advice.

  • 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.

Nowadays, it is becoming more and more common to opt for a single embryo transfer. The new embryo selection techniques make it possible to choose those embryos of better quality and, therefore, with a greater probability of resulting in a pregnancy.

Those surplus embryos that are not transferred can be vitrified for future transfer. Other options would be to donate them to other couples or to research.

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.

Get more information on frozen embryo transfer here: Frozen Embryo Transfer (FET) in IVF.

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.

Once the endometrium is ready, the embryo is transferred.

Read more about medication protocol before embryo transfer: Endometrium Preparation for Embryo Transfer: How does it work?

Embryo transfer step by step

As stated before, embryo transfer 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 catheter is checked in the laboratory.

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 center itself. Afterwards, she can go about her everyday life.

Tips for transfer day

When the endometrium of the surrogate is receptive and has acquired the ideal characteristics that favor embryo implantation, she will be scheduled to transfer the embryos. Here, we have some tips on how to prepare for the embryo transfer day:

  • Having a full bladder to facilitate catheter insertion.
  • Being relaxed avoiding muscle contraction.
  • No perfumes, creams or other chemicals that may harm the embryos.

On the other hand, it is not necessary for the woman to come to the transfer on an empty stomach since she will not be anesthetized.
Furthermore, the specialist will also recommend continuing with the daily routine, avoiding excessive efforts and intense exercise after transferring the embryos to the uterus. Other advice for the surrogate will be to drink a lot of water, avoid immersion baths, and not have sexual relations.

Post embryo transfer symptoms

Symptoms and discomfort after an embryo transfer can be different in each woman. Normally, these clinical manifestations are usually derived from the medication administered for the endometrial preparation. What is important to know is that the presence or absence of symptoms from the time the embryos are transferred until the pregnancy test is performed is not indicative of the success or failure of the treatment.

Listed below are the most common symptoms described by some women after embryo transfer:

  • Abundant vaginal discharge.
  • Bleeding of low intensity and duration.
  • Changes in the breasts.
  • Nausea.
  • Increased urge to urinate.

It is advisable to consult a physician if the patient notices any discomfort of great intensity and/or if it is prolonged over time, in order to know how to proceed.

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.

Can I have sex after embryo transfer?

By Marta Barranquero Gómez (embryologist).

Most specialists do not recommend sexual intercourse after an embryo transfer, since uterine contractions could affect embryo implantation.

However, there are some studies, although few, that indicate that sexual intercourse may be beneficial in stimulating immune tolerance. Therefore, the usual tendency of most fertility centers is to discourage sexual intercourse.

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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