ObjectiveTo systematically review the association between C677T and A1298C polymorphisms of methylenetetrahydrofolate reductase (MTHFR) gene and the risk of unexplained recurrent spontaneous abortion (URSA). MethodsWe searched PubMed, EMbase, CBM, CNKI, VIP and WanFang Data from inception to May 2015 to collect case-control studies about the association between the MTHFR gene C677T and A1298C polymorphisms and the risk of URSA. Two reviewers independently screened literature, extracted data and assessed the risk of bias of included studies. Then, meta-analysis was performed using RevMan 5.0 and Stata 12.0 software. ResultsA total of 42 case-control studies involving 3 970 URSA patients and 5 297 controls were included. The results of meta-analysis showed that MTHFR C677T polymorphism was associated with the increased risk of URSA (T vs. C: OR=1.34, 95% CI 1.16 to1.54, P < 0.000 01; TT vs. TC+CC: OR=1.70, 95% CI 1.36 to 2.12, P < 0.000 01; TT+TC vs. CC: OR=1.34, 95% CI 1.11 to 1.62, P=0.002; TC vs. CC: OR=1.19, 95% CI 0.99 to 1.43, P=0.061; TT vs. CC: OR=1.95, 95% CI 1.48 to 2.56, P < 0.000 01). Subgroup analysis by ethnicity indicated that the MTHFR C677T polymorphism was associated with the increased risk of URSA in east Asians (T vs. C: OR=1.61, 95% CI 1.39 to 1.87, P < 0.000 01; TT vs. TC+CC: OR=2.05, 95% CI 1.54 to 2.71, P < 0.000 01; TT+TC vs. CC: OR=1.76, 95% CI 1.41 to 2.19, P < 0.000 01; TC vs. CC: OR=1.53, 95% CI 1.21 to 1.94, P < 0.000 01; TT vs. CC: OR=2.77, 95% CI 1.94 to 3.97, P < 0.000 01) but was not associated with the increased risk of URSA in Caucasians. The results of meta-analysis also showed that there was no significant association between the MTHFR A1298C polymorphism and the URSA in all population. ConclusionCurrent evidence indicates that significant association is found between MTHFR C677T mutation and URSA in east Asians but not in Caucasians. Further study indicates that women carrying TT or TC gene significantly increases the risk of URSA and TT mutant gene carriers have a higher URSA risk. There is no significant association between MTHFR A1298C mutation and URSA in all population. Due to the quantity and quality limitations of included studies, more high quality case-control or cohort studies are needed to verify the above conclusions.