The main purpose of the study is to evaluate an OGTT at 18 to 20 gestational weeks as an early diagnosis method of GDM in low risk pregnant women. By investigating maternal and neonatal outcomes that are associated with the early OGTT and regular OGTT results, the study will provide a evidence for the most appropriate time to perform an OGTT in pregnant women.

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.