**14. Conclusions**

Although both a viable blastocyst and appropriately primed endometrium are necessary for successful implantation/placentation, we posit that the optimal priming (decidualization) of the endometrium is the most critical determinant of pregnancy success. Decidualization involves the recruitment of leukocytes and, critically, the genetic reprogramming of ESCs to DSCs that suppress inflammation, promote vascular adaptation, and foster tolerance to fetal antigens. Since decidualization in humans occurs during every menstrual cycle in the absence of the conceptus, pregnancy health is determined even before the blastocyst arrives. A better understanding of the molecular and cellular mechanisms responsible for endometrial decidualization will improve our understanding of the factors influencing pregnancy success and provide opportunities for novel approaches to diagnosis and treatment.

**Author Contributions:** Conceptualization, S.-W.N., M.P., T.T., C.S., and E.R.N.; writing—original draft preparation, S.-W.N., G.A.N., and E.R.N.; writing—review and editing, M.P., T.T., and C.S.; visualization, S.-W.N., G.A.N., and E.R.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** Carlos Simón is Founder and Head of the Scientific Advisory Board of Igenomix.
