Embryo transfer

What is embryo transfer?

Embryo transfer is the transfer of embryos into the uterus, the final step of an in vitro fertilization (IVF) program. All steps such as examination, ovarian stimulation, egg retrieval, sperm preparation, fertilization, and culture are preparatory steps for embryo transfer. After a successful embryo transfer, all that remains is to wait for the result.

Types of embryo transfer

There are two main types of embryo transfer: fresh embryo transfer and frozen embryo transfer. The latter option does not exactly describe the process, as it is not a transfer of a frozen embryo, but rather a pre-thawed embryo that has been cryopreserved. This transfer is also called cryopreservation or embryo transfer in a cryoprotocol.

Fresh embryo transfer

Fresh embryo transfer has the advantage of minimizing the time to conception. Once the egg is retrieved from the ovary and fertilized, only a few days pass before the transfer (commonly on day 5), and then one must wait two to three weeks to know the result through a pregnancy test.

The main disadvantage of fresh transfer is the possibility of endometrial issues and the matching between the age of the embryo and the development of the endometrium. The hormones used to stimulate ovulation and produce eggs can slow down the growth of the endometrium. It is important for the endometrium to have a thickness of at least 7 to 8 mm at the time of transfer, which provides the best conditions for successful embryo implantation. Fresh transfer may be discouraged for women at high risk of ovarian hyperstimulation syndrome, which reduces chances of success.

Frozen embryo transfer (cryopreservation)

Cryopreservation has the advantage of being able to delay the transfer to any time, allowing the woman's body to recover from the egg retrieval and arrive at embryo transfer under optimal conditions. Delaying the transfer until the body has fully recovered helps avoid the mismatch of the embryo's age and the readiness of the endometrium, as the embryo is thawed at a time when the endometrium has reached its optimal maturity.

Another important advantage of frozen embryo transfer is the possibility of performing preimplantation genetic testing (PGT). With this test, doctors can select the highest quality embryo, excluding genetic abnormalities, which significantly increases the chances of success.

The disadvantage of cryopreservation is its high cost. However, in the long run, it may prove more advantageous, as embryos are often frozen in advance. In case of failure or if a couple wishes to have another child, there is no need to go through all the complicated steps of preparation again, as there are already ready-to-use embryos that can be thawed and transferred at any time.

Three and five-day embryo transfer

The embryo is transferred to the uterus on the third or fifth day after the fertilization of the egg, which is an important factor as an embryo at such an early stage grows and changes rapidly.

Once the cells of the mother and father have fused, a zygote is formed, and the cells begin to divide. Two cells form on the first day, four cells on the second day, and eight cells on the third day. The embryo must have at least four and preferably eight cells to be transferred to the uterus. On the fourth day, the zygote transforms into a morula, and on the fifth or sixth day into a blastocyst, which consists of several dozen cells (60-120). These cells are already differentiated: some will transform into a fetus, others into placenta.

The main advantage of transferring on the third day is that only one-third of embryos survive to the fifth day. Therefore, if the number of embryos is limited, reproductive doctors prefer to transfer embryos on the third day to avoid losing a chance.

However, a five-day embryo has a higher chance of successful implantation, being genetically healthy, avoiding premature pregnancy, and giving birth to a living child. By diagnosing embryos at the blastocyst stage, the embryologist has a better choice, which is why most specialists prefer a transfer on the fifth day if possible.

The process of embryo transfer within IVF

The woman arrives at the clinic at the scheduled time. The embryo transfer is performed in the gynecological chair under ultrasound control. The gynecologist first determines the length of the cervical canal using ultrasound, then prepares the cervix. The embryo is placed by the biologist into a thin disposable catheter that resembles a syringe with a long thin tube instead of a needle. The gynecologist inserts the catheter into the uterus. The passage of the catheter through the cervix may cause slight discomfort. Using ultrasound, the doctor determines the best location and releases the embryo into the uterine cavity. The catheter is then checked by the biologist to ensure that the embryo has not remained inside it. The embryo transfer is then completed, and the woman is advised to lie down for a while to rest and relax. The entire procedure, including preparation and rest, takes about two hours.

What is the day of embryo implantation after transfer?

The implantation of the embryo into the endometrium, that is, its attachment to the uterus, occurs between 6 and 10 days. A five-day embryo is generally implanted on the seventh day. The process occurs in two stages: the embryo first adheres to the surface of the endometrium (adhesion), and then it sinks into its thickness. This process takes about 40 hours and does not cause any particular sensations in the woman. It is unnecessary to take pregnancy tests immediately after the transfer, as at this stage the result will not be reliable.

Following the transfer, tracking the development of the embryo day by day is difficult, as implantation occurs at different times. That is why gestational age and the level of development of the embryo are assessed in weeks.

What happens after embryo transfer and when to diagnose pregnancy?

After the transfer, the embryo continues to divide, and after implantation, the formation of the placenta and the body of the fetus begins. The fetal egg is covered with a membrane, the chorion, which attaches to the wall of the uterus. At the site of attachment, the chorion has villi that ensure close contact with the endometrium, and it is at this location that the placenta forms.

Within the first hours following implantation, the chorion begins to produce a hormone, human chorionic gonadotropin (hCG), which signals to the body the onset of pregnancy. The level of hCG rises like an avalanche and can be detected by laboratory tests 7 to 10 days after implantation. Therefore, pregnancy can be diagnosed 13 to 20 days after embryo transfer.

Double embryo transfer

Double embryo transfer is an IVF procedure in which embryos are transferred twice during a cycle. In the case of asynchronous double transfer, one embryo is transferred first and the second embryo is transferred 2 to 3 days later, which increases the chances that one of the embryos reaches the implantation window.

Success indicators

Double embryo transfer can increase the chances of a successful pregnancy in patients who have experienced multiple failures and whose implantation window is shifted. However, one must consider the risk of multiple pregnancies, which is about 29% in the case of double transfer, compared to 2% in the case of single transfer. Multiple pregnancies are associated with an increased risk of complications such as premature labor, low birth weight, the need for a cesarean section, and an increased risk of congenital malformations.

Single embryo transfer

In recent years, the trend has been towards the selective transfer of a single embryo (SET) due to the potential risks associated with multiple pregnancies. Thanks to advances in IVF technology, doctors can create high-quality embryos, making single embryo transfer a good option for many patients. Young women generally have better quality eggs, and selective single embryo transfer is therefore the preferred option for them. If the embryos are of high quality, SET is also preferable.

Double embryo transfer may be a choice for some patients, but it is important to discuss all these options with your doctor to choose the best solution for your personal situation.