Endometrial Vitamin D Signaling and Immune Escape in Recurrent Pregnancy Loss

Recurrent pregnancy loss (RPL) continues to be a significant challenge in reproductive medicine, particularly in women for whom standard examinations do not reveal a conclusive underlying reason. There is growing evidence that several instances may result from nuanced alterations in the endometrial milieu, particularly with decidualization and maternal–fetal immune tolerance. In recent years, vitamin D has gained recognition for its significance in early pregnancy, serving not only as a regulator of calcium metabolism but also as an active contributor to endometrial and immunological functions. The human endometrium exhibits the vitamin D receptor (VDR) and the enzyme CYP27B1, facilitating the local activation and signaling of vitamin D inside the uterine milieu. Experimental investigations have shown that vitamin D influences many processes critical for effective implantation and placentation, including stromal cell differentiation, cytokine equilibrium, trophoblast invasion, oxidative stress responses, and immune cell communication. Aberrant vitamin D signaling has been associated with heightened inflammatory activity, impaired decidual transformation, altered uterine natural killer cell functionality, and alteration of the Treg/Th17 equilibrium, all of which have been implicated in recurrent pregnancy loss. Concurrently, there is an increasing emphasis on the association between vitamin D and mitochondrial function as well as oxidative stress in decidual and endometrial cells. Interruption of these pathways may influence implantation and early embryonic development by impacting cellular metabolism and immunological control at the maternal–fetal interface. The clinical interest in vitamin D supplementation for women experiencing repeated reproductive failure is increasing; nevertheless, the existing results are conflicting, mostly due to the predominance of research focusing on circulating vitamin D levels rather than localized tissue-specific processes. Our review encapsulates new findings about the function of vitamin D in endometrial biology and reproductive immune regulation, emphasizing its involvement in decidualization, inflammatory signaling, oxidative stress, and maternal–fetal immunological tolerance in recurrent pregnancy loss. The potential ramifications for assisted reproduction and forthcoming tailored therapy techniques are also examined.