Objective To investigate the role of Sneathia sanguinegens(S.sanguinegens)in the development of unexplained recurrent spontaneous abortion(URSA).Methods A case-control study was conducted to analyze the vaginal flora characteristics of 65 patients with URSA and 18 healthy controls through 16S rRNA gene sequencing.Toxicity profile of S.sanguinegens on human cervical cancer cells(ME-180),human umbilical vein endothelial cells(HUVEC)and human placental choriocarcinoma cells(JEG-3)was analyzed at the cellular level to assess the mechanism of it in adverse pregnancy outcomes.And S.sanguinegens was used to infect C57BL/6J mice to explore the toxic effect on living organisms.Results The relative abundance of Sneathia was increased in patients with URSA compared with healthy controls.It was positively correlated with the number of miscarriages,and was attributed to S.sanguinegens.We also found that S.sanguinegens damaged ME-180,JEG-3 and HUVEC cells.The degree of cellular damage was related to the level of S.sanguinegens added.Intravenous infection with S.sanguinegens caused inflammatory damage in several organs and extramedullary hematopoiesis in the spleen.Conclusion S.sanguinegens is closely related to URSA and should be emphasized in patients with high vaginal bacterial load.