The objectives of this study were to determine the incidence of gestational diabetes mellitus (GDM) before 20 weeks of gestation and evaluate associated risk factors. A total of 1200 pregnant women who started their antenatal care at Siriraj Hospital before 20 weeks of gestation and were at risk of developing GDM were enrolled. A 50-gram glucose challenge test was used as a screening method and 100-gram oral glucose tolerance test was used to diagnose the condition. All women were screened during their first visit and again at 28-32 weeks of gestation. The results showed that the incidence of GDM diagnosed before 20 weeks of gestation was 5.3% (95% CI 4.1-6.7%), and another 4.9% were diagnosed during 28-32 weeks. This demonstrated the usefulness and effectiveness of the current guideline for GDM screening that approximately half of women with GDM could be diagnosed early in their pregnancies. The 2 independent risks for developing GDM before 20 weeks of gestation were age > or = 30 years and GDM in previous pregnancy (adjusted OR 2.5, 95% CI 1.3-5.0, and 7.1, 95% CI 1.8-27.6, respectively). This specific group of women should be counseled regarding the importance of early GDM screening during their pregnancy.