Embryo transfer in the United States

The embryo transfer is a stage of in vitro fertilization (IVF) in which the embryos that have developed in the laboratory are placed inside the maternal uterus, waiting for them to attach adequately to the endometrium and give raise to a new pregnancy.

The endometrium is the inner mucous membrane that covers the uterus and the place where embryo implantation and subsequent fetal development throughout the nine months of pregnancy take place.

Embryo attaching itself to the uterine lining

The success of the transfer and the happy ending of the fertility treatment therefore depend not only on embryo quality, but also on endometrial receptivity. Thus, for the embryos to be capable of attaching to the uterus, the endometrium must:

• Have a trilaminar pattern
• Have an average thickness of around 7-10 mm

It is a two-way communication process: on the one hand, the embryos and, on the other hand the uterus. For pregnancy to take place, maintaining the flow of communication among them is crucial.

How to have a problem-free pregnancy

Theoretically, every single embryo transferred should be able to implant. Unfortunately, not all of them may be capable of doing so; therefore, a three-embryo transfer does not necessarily imply the woman is going to have a multiple birth, though it makes it more likely for at least one of them to implant.

Deciding how many embryos to transfer is not an easy decision to make, since two factors come into conflict:

• Chances of getting pregnant
• Risks associated with multiple births

One should keep in mind the aim sought, i.e. becoming pregnant, without overlooking the potential risks a multiple pregnancy entails. This is the reason why many countries have established limits regarding the maximum amount of embryos to transfer. In the United States, however, no limited is set by law, thereby giving doctors complete freedom to decide it.

Risks associated with multiple births

Certain specialists have a preference for focusing on the ultimate goal of the treatment (i.e. pregnancy), thereby playing down the potential risks involved both for the pregnant and the unborn babies. On the other hand, others prefer to find the right balance between both factors before determining how many embryos to transfer.

Things to consider before the embryo transfer

Another aspect that should be kept in mind before the transfer is the day when it will be performed. In general, there are two possible options:

• Embryo transfer on day 2-3 of embryo development
• Embryo transfer on day 5-6 of embryo development

There is also the possibility of performing the embryo transfer on day 4 post ovum pick-up, although this is a less common option because by that time it is at its morula stage of development.

Depending on embryo quality and quantity, endometrial receptivity, and the need for additional procedures such as PGD, the embryos will will be transferred one day or another.

Finally, special attention should be also drawn to embryo cryopreservation. In this sense, the transfer can be done in two ways:

• A fresh embryo transfer: The egg-sperm binding takes place in the laboratory, thereby creating the embryos. Then, after a certain period of culture, some embryos are selected for the transfer and the unused embryos are cryopreserved for prospective transfers.
• Freeze-thaw process: the embryos obtained are frozen generally through embryo vitrification. Once the embryo transfer is scheduled, the embryos will be defrosted and transferred to the maternal womb.