Gestational diabetes mellitus (GDM) is described as any degree of glucose intolerance first recognized during pregnancy. Women with GDM and their offspring have a risk to develop Type 2 diabetes mellitus in later life. For diagnosis of GDM, criteria such as ADA, WHO, DIPSI etc., are followed. In 2013 WHO reformed its criteria for GDM, followed by development of IADPSG criteria. In this study an attempt was made to compare these criteria with an emphasis to study the role of HbA1C in the diagnosis of GDM and overt diabetes during pregnancy. Methods: 289 pregnant women were screened for GDM by 75 g OGTT in fasting, and HbA1C during first ANC visit and at 24 to 28th week of gestation. Women with thyroid and other disorders were excluded. Relevant clinical information was collected. Plasma glucose was analyzed by autoanalyzer and HbA1C by NGSP certified method. SPSS 24 was used for statistical analysis Results: 13.6, 18.3, 32.9, and 31.2 percent prevalence of GDM respectively was observed according to DIPSI, WHO 1999, IADPSG and WHO 2013 criteria. Applying HbA1C cutoff values (%) of > 6.5, and 6.0 – 6.49, there were 6 (2.0%) with diabetes and 11 (3.8%) subjects with impaired glucose tolerance respectively. Approximately 4% women were found to be suffering from overt diabetes based on both HbA1C and FPG values as recommended by IADPSG. Conclusion: Screening with HbA1C along with OGTT for the diagnosis of GDM would be a significant marker to rule out pre–existing or overt diabetes in pregnant population and calculation of reliable existing prevalence rate of GDM in pregnant population