The Early Diagnosis of Gestational Diabetes Mellitus Study

Inclusion Criteria: * Singleton pregnant women will be invited to participate in this study if they are between 18 and 40 years of age, and have had their first prenatal visit in our center before 18 weeks 0 days of gestation. Exclusion Criteria: * Previous history of GDM or preexisting DM * Family history of diabetes mellitus (first degree relative with diabetes or a sister with GDM) * Body mass index\>30 kg/m2 * Previous macrosomia (baby with birth weight \>4,000 g) or a history of stillbirth * Polycystic ovary syndrome * Medications: corticosteroids, antipsychotics * Participant not willing to take OGTT twice (at 18-20 gestational weeks and 24-28 gestational weeks), or not willing to have a series of prenatal care visits and deliver in our center.