Insights into embryo implantation in humans

The majority of all preimplantation embryos fails to implant into the endometrium of the uterus. In this thesis, we describe our efforts to better characterize both embryonic and endometrial features, and their respective functions in embryo-endometrial interaction during implantation in humans.
In a clinical context, we demonstrated that combining two embryos during embryo transfer, regardless of the quality of the embryos, does not affect pregnancy rates, but only results in higher multiple gestation rates. Importantly, we also showed that transferring a high-quality embryo from a cohort containing at least one more high-quality embryo, leads to higher pregnancy rates compared to transferring a high-quality embryo from a cohort of otherwise low-quality embryos.
Regarding embryo-endometrial interaction, we demonstrated that high-quality preimplantation embryos, triggered by the secretion of specific factors, are capable of recruiting decidualized endometrial cells, while at the same time rejecting non-decidualized cells. By doing so, a high-quality embryo seems capable of optimizing its own implantation potential. We further demonstrated that low-quality embryos, which secrete a different subset of factors, fail to properly select these endometrial cells, resulting in defective implantation. Moreover, in case of failed implantation, we showed that the endometrium is often prematurely decidualized during the luteal phase, leading to dysfunctional embryo-endometrial interaction, and consequently implantation failure.
Conjointly, this thesis provides insights into crucial aspects of implantation in humans, that may ultimately help to enhance treatment efficacy and thereby pregnancy rates in IVF.