The Impact of the Number of Implantation-Window uNK Cells on Pregnancy Outcomes and Decidualization

This study aims to further clarify the correlation between the quantity of natural killer (NK) cells in the endometrium and the successful implantation of embryos during the specific implantation window. Patients were stratified into four distinct groups according to the quantity of natural killer (NK) cells detected during the implantation window. The groups were defined as follows: Group A (NK cell percentage ≥ 10%, n = 62), Group B (NK cell percentage: 5%-9.99%, n = 128), Group C (NK cell percentage: 1%-4.99%, n = 106), and Group D (NK cell percentage ≤ 0.5%, n = 30). Among Group A, 32 patients received intrauterine infusion of dexamethasone (designated as Subgroup A1), while the remaining 30 patients did not undergo this treatment (designated as Subgroup A2). For Groups B and D, immunohistochemistry was performed to detect the expression of IGFBP1 (an endometrial decidualization marker) and HBP1 (a decidualization-related transcription factor). The live birth rate in Group D was statistically significantly lower than that in the other three study groups. For Group A, after dexamethasone administration, a notable reduction in NK cell quantity was observed in Subgroup A1 (patients who received intrauterine dexamethasone infusion) during the implantation window; however, the pregnancy rates of Subgroup A1 and Subgroup A2 (patients without dexamethasone treatment) were comparable. Furthermore, in Group D patients, the expression levels of the decidualization markers IGFBP1 and HBP1 were significantly lower than those in the reference group (Group B, as specified in Methods). Insufficient natural killer (NK) cells during the embryo implantation window significantly impede the embryo implantation process. This shortage of NK cells may be attributed to inadequate endometrial decidualization. Notably, although intrauterine infusion of dexamethasone effectively reduces NK cell counts, it does not exert a significant impact on clinical outcomes.