When Is the Embryo Transfer Performed?

The embryo transfer is the last stage of the in vitro fertilization (IVF) process. Once the embryos have been transferred to the gestational carrier's uterus, the work of the professionals in assisted reproduction culminates and the success of the process depends solely on the embryos themselves.

In this article we will comment on some aspects related to the transfer that may influence its success.

Decisions regarding the transfer

There are two main questions related to embryo transfer:

• Number of embryos to transfer
• On which day of embryonic development to perform the transfer

The number of embryos to be transferred to the surrogate in a surrogacy process is a decision to be made by consensus between the parents and the surrogate, but always considering medical advice. However, the most appropriate moment to make the transfer is determined by the embryologists, although the patients are always informed.

This last aspect has generated and continues to generate to this day great debate as there are advantages and disadvantages in both day 3 and day 5. Therefore, the decision must be taken individually, taking into account the characteristics of each situation.

Optimal time for transfer

The best time to transfer the embryos is determined by the endometrial receptivity. The final objective of the transfer is to get the transferred embryos to implant, so the endometrium must be prepared for this at the moment when the embryos are introduced into the pregnant woman's uterus.

The embryos can be transferred on either day 2/3 or day 5-6 (blastocyst). Day 4 is discarded because it is the moment when the embryos are in the morula stage, which does not allow their characteristics to be clearly evaluated.

In addition to the receptivity of the endometrium, there are other aspects to be taken into account: the quality of the embryos, the number of viable embryos obtained, whether preimplantation genetic diagnosis (PGD) is needed, and the clinical history of each patient.

Viable Embryos

A multiple pregnancy (twins, triplets...) is a higher risk pregnancy than a single pregnancy so assisted reproduction seeks to improve embryo selection with the aim of transferring a single embryo and thus reduce the probability of a twin pregnancy.

The transfer on day 5-6 may allow us to more accurately select the embryos with the greatest potential. However, there is a possibility of running out of embryos to transfer, one of the main fears that lead to the transfer on day 3.

Embryos for transfer

Although it is possible that the culture up to day 5 (blastocyst stage) will improve the selection in many cases, in general, it is recommended to transfer on day 3, especially when there are few embryos. The development of the embryos from day 3 to day 5 in the laboratory is delicate and leaving them in the incubator for more days may imply blocking of growth and therefore the cancellation of the transfer.

The laboratory conditions and the quality of the facilities are very important when deciding whether to leave the embryos in culture until the blastocyst stage. The laboratory must have the appropriate means that allow the adequate evolution of the embryos until the moment they are going to be transferred.

Pre-implantation genetic diagnosis

When there is a family history of genetic disease or the future parents present some alteration in their genes, it is recommended to perform PGD. Also in cases of advanced maternal age, repeated implantation failures or recurrent abortions are indications for PGD.

The main objective of PGD is to discard for transfer those embryos with some genetic alteration, that is, susceptible to develop some serious disease.

In order to genetically analyze an embryo, it is necessary to extract one of its cells and evaluate its genetic composition. This procedure takes between 24 and 48 hours. In order to be able to extract an embryonic cell without altering its development, it is necessary that between 6 and 8 cells exist. For this reason, in IVF cycles with PGD, the transfer is necessarily carried out on day 5.