Gestational diabetes mellitus (GDM) is defined as carbohydrate intolerance of varying degrees of severity with onset or first recognition during pregnancy. GDM is caused by the inability to overcome diabetogenic state due to insufficient pancreatic function. It carries significant and often potentially grave maternal and fetal complications including preeclampsia, polyhydramnios, fetal macrosomia, birth trauma, operative delivery, neonatal metabolic complications and perinatal death. Development of obesity and diabetes in offspring during childhood and later development of diabetes mellitus in the mother are also related with GDM. Therefore, accurate screening and early diagnosis of this condition is very important to enable timely intervention in order to ensure a satisfactory pregnancy outcome(1). The prevalence of gestational diabetes varies worldwide and among racial and ethnic groups(2). It also varies with the testing methods and diagnostic criteria. The increase in prevalence of GDM was recently reported to be an artifact caused by universal screening, with no evidence of benefit of this scheme to pregnancy outcome(3). At Maharaj Nakorn Chaing Mai Hospital, glucose challenge test (GCT) screening by risk factors and confirmed by glucose challenge test (GCT) strategy has been practiced for many years without extensive evaluation. Therefore, the present study was conducted to determine the prevalence and risk factors of GDM and its impact on maternal and perinatal outcomes in the population of both screening and non-screening groups.