Embryos and implantation: The Good, the Bad, and the Ugly

Last month we have closed the year with a piece about the role of the endometrium in selecting which embryo will implant, which generated a lot of interest, so much so that a few of you contacted us wanting to discuss more in detail the issue. Well, let’s listen to your interest and greet the New Year with more implantation talks!

A question that we often face in our daily practice is what to do when we have only a couple of embryos on the day of transfer, and one of them is of poor quality. What do we do? Should we transfer it anyway, because a bad embryo is better than no embryo at all? Or should we perhaps not transfer it, because the biochemical signals that it will release will diminish the implantation potential of both embryos?

Over the years, a few mathematical models have been developed to identify if two embryos transferred at the same time act independently one from the other when they implant, or if their have an interaction, and it has been established clearly that the independent model does not fit the data from thousands of transfers. Indeed, when embryos are transferred together, the implantation rate for each of them is higher than what you would find if you were to transfer each of them separately, indicating that a collaborative or group model are best suited to explain the empirical observation (Matorras et al, 2005; Williams et al, 2012).

However, all these studies were carried out assuming that all embryos have the same, average, implantation potential, while we know of course that the implantation potential of all embryos is not equal, and it can, indeed, be very different among them. It might be that the pattern of implantation is different when transferring 2 average quality embryos vs. transferring one high quality + 1 low quality, even if the resulting average implantation rate is the same.

To date, the most readily available marker of implantation potential is embryo morphological score. Although it is true, of course, that a there are embryos with high score that will not implant, and that perfectly viable pregnancies can be obtained with embryos with low score, in general, the higher the morphological score of an embryo, the higher its chances to implant. So, going back to the original question, are we doing any good in terms of pregnancy and implantation rate by transferring a low quality embryo to “accompany” a good quality one?

A recent report (Matorras et al, 2013) suggests that, when taking into account embryo quality (defined as good, fair, or poor), the synergy between transferred embryos remains evident, however, its magnitude changes very significantly, in a way that it is tied to embryo quality.

As an example, if we transfer an embryo with an implantation potential of 40% (meaning that the pregnancy rate in a SET would be 40%), and we add to it another embryo with an implantation potential of also 40%, we’ll have a pregnancy rate of 64%. However, this significant increase in pregnancy rate will be accompanied by an increased in multiple pregnancy from 0% to 24%, because so to speak the 2 good embryos, “help” each other a lot.

However, if we add to the first embryo a second one of low quality, with an implantation potential of 10%, like the case presented at the beginning, pregnancy rates will also slightly increase, from 40% to 46%, and twin pregnancies will go from 0% to 6%.

It is then clear that if we have an embryo of poor quality, and we do not know what to do, transferring it will not harm the chances of pregnancy. However, we should be careful in our attempts to improve indefinitely pregnancy rates by transferring multiple high quality embryos, as the relative increase in pregnancy rate will be almost entirely be provided by the multiple pregnancies that will be generated.

As always, understanding of the respective model of implantation for each embryo combination, and careful counselling to understand the wishes and the needs of our patients will be key factor in deciding the best transfer policy in our practices.

Rita Vassena
Scientific Director, Clinica EUGIN