Introduction: Gestational Diabetes Mellitus (GDM) is a common metabolic disorder in pregnancy that is diagnosed for the first time in the second half pregnancy. Associations between consumption of trans fatty acids and GDM have been conflicted in manystudies. The purpose of this study is to determine this relationship. Materials and Methods: This is a narrative review to assess the relationship between trans fatty acids consumption before and during pregnancy and GDM incidence. We searched Google Scholar, Cochrane, and Science Direct and Med Line for identifying relevant analytic studies (Cohort and case- control studies). We used the key words: Trans Fatty Acids, Gestational Diabetes Mellitus, pregnancy glucose intolerance. Result: At last we identified 12 relevant studies. Studies demonstrated that high trans fatty acids and animal fat intake before and during pregnancy significantly increased blood glucose levels in late pregnancy. Conclusion: High trans fatty acids intake and animal fat consumption in daily diet before and during pregnancy disturb glucose metabolism and increase risk of GDM in the second half of pregnancy, although all of the studies recommended more widespread clinical trialsto identify the exact relationship between high trans fatty acids intake and GDM.