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Study Question: Is there an association between hereditary thrombophilia in pregnant women and risk of recurrent pregnancy loss (RPL)?Summary Answer: Pregnant women with hereditary thrombophilia have an increased risk of RPL, especially for pregnant women with the G1691A mutation of the factor V Leiden (FVL) gene, the G20210A mutation of the prothrombin gene (PGM), and deficiency of protein S (PS).What Is Known Already: Prior studies have suggested that pregnant women with hereditary thrombophilia have a higher risk of RPL, however, the results are inconsistent; furthermore, a complete overview is missing. This lack of information is an obstacle to the risk assessment of RPL in pregnant women with hereditary thrombophilia. A comprehensive meta-analysis on the relation between hereditary thrombophilia and the risk of RPL is needed.Study Design, Size, Duration: A systematic review and meta-analysis was performed using observational studies published in English before 1 April 2020 to evaluate the relation between hereditary thrombophilia and risk of RPL.Participants/materials, Setting, Methods: Relevant studies were identified from PubMed, Web of Science, and EMBASE searches and complemented with perusal of bibliographies of retrieved articles. The exposure of interest was hereditary thrombophilia, including FVL mutation, PGM, deficiency of antithrombin (AT), deficiency of protein C (PC), and deficiency of PS. The overall risk estimates were pooled using random effects models. Subgroup and sensitivity analyses were carried out to explore possible sources of heterogeneity and assess the robustness of the results.Main Results and the Role Of Chance: A total of 89 studies involving 30 254 individuals were included. Results showed that women with FVL mutation (odds ratio (OR): 2.44, 95% CI: 1.96-3.03), PGM (OR: 2.08, 95% CI: 1.61-2.68), or deficiency of PS (OR: 3.45, 95% CI: 1.15-10.35) had higher risks of developing RPL. Compared with the reference group, there was no observed relation between a deficiency in AT or PC and RPL (all P > 0.05). Heterogeneity in the risk estimates of RPL was partially explained by geographic region, definitions of RPL, types of RPL, and controlled confounders. Sensitivity analyses validated the robustness of the findings.Limitations, Reasons For Caution: Only 39 of the included studies controlled for one or more confounders, and the heterogeneity across all included studies was high. Based on the data available, we cannot determine whether this association is confounded by other potential risk factors of RPL.Wider Implications Of the Findings: This systematic review and meta-analysis show a possible association between hereditary thrombophilia and an increased risk of RPL, suggesting that testing for hereditary thrombophilia should be considered in individuals with RPL.Study Funding/competing Interest(s): The study was funded by the Hunan Provincial Key Research and Development Program (Grant number: 2018SK2062) and National Natural Science Foundation Program (Grant number: 81973137). There are no conflicts of interest.Registration Number: N/A. [ABSTRACT FROM AUTHOR]