This question has a simple starting answer, but a complex background. Implantation, in a nutshell, is the act of an embryo invading and establishing itself within the lining of a woman’s uterus. In this crucial act, the fate of a pregnancy could be decided. To understand it, we must delve into the background. Namely, what is needed of an embryo and what is needed of the lining? Still, both questions remain incompletely answered.

What is needed for an embryo to implant?

As far as the embryo is concerned, a few key facts must hold. First is that an embryo needs to be at least partly chromosomally normal. We say “partly” because embryos with a mix of chromosomally normal and chromosomally abnormal cells (called mosaic embryos) can still implant and lead to a live birth around 30% of the time. Second, we know that the embryo must express the right proteins to bind to the cells in a woman’s lining. Once the embryo finds a spot that is hospitable and binds to it, it can start interacting with and invading around the cells within this lining.

Two key points should be noted here:

1. We know that normal chromosomes do not guarantee that an embryo will accomplish these actions,
2. We don’t have any reliable and safe tests to determine the embryo’s ability to interact and invade before we do an embryo transfer.

What is needed of the uterine lining?

Concerning the lining, we know that each woman has a “window-of-implantation.” This is the short span of time between cycle days 19 to 24 when the lining is most likely to support the implantation of an embryo. This is the period when the lining is most ‘receptive’. A key factor involved is progesterone. At least several days of this hormone is required to achieve receptivity. This is based upon numerous studies looking at pregnancy rates with the transfer of embryos under different amounts of progesterone exposure. In general, the time of progesterone exposure should roughly correspond to the gestational age of the embryo. The two should be synchronized.

What is more important to implantation?

One burning question at this point is what is more important to implantation, the embryo or the lining. It could be argued that it is the embryo.  If the lining was so crucial then embryos would be able to implant solely into ideally-prepared uterine linings. This would mean that ectopic pregnancies should not be possible, but yet they do happen. However, as mentioned above, if a lining is not properly treated with progesterone, then implantation rates also decrease. Most likely it is a combination of both the embryo and the lining.

The great unknown and beyond

Could there potentially be a third factor involved that we do know of. That is very possible since even the best embryos transferred into thick, lush, progesterone-soaked linings still cannot implant. Other factors such as a hidden infection /inflammation (called chronic endometritis) or changes in endometrial genetic activity, among others, have been implicated in implantation failure.

What is definite is that the vast majority of patients will eventually have an embryo that will implant and succeed – it’s just a question of when and how persistent the patient-doctor team wish to be. Ongoing research will clarify many of the above questions and, hopefully, we’ll reach a day when every normal embryo consistently leads to a pregnancy, and a healthy live birth.