The significance of an early diagnosis of gestational diabetes mellitus(GDM) with oral glucose tolerance test (OGTT) has not been determined.To investigate GDM diagnosed by early and standard OGTTs and determine adverse maternal and neonatal outcomes associated with early GDM diagnosis.The Early Diagnosis of Gestational Diabetes Mellitus study is a prospective cohort study. Each participant in the study undertook two OGTTs, an early OGTT in 18-20 gestational weeks(gws) and a standard OGTT in 24-28 gws. The reproduciblity between early and standard OGTT were analysed. Maternal and neonatal metabolic disorders and pregnancy outcomes were compared across groups.A total of 522 participants completed both the early and standard OGTTs. The glucose values in the early OGTT were not significantly different from those in the standard OGTT (Fasting: 4.31±0.41 mmol/L vs. 4.29±0.37 mmol/L, P=0.360; 1hour: 7.68±1.71 mmol/L vs. 7.66±1.59 mmol/L, P=0.826; 2hour: 6.69±1.47mmol/L vs. 6.71±1.39mmol/L, P=0.800). The reproducibility of early and standard OGTT results was 74.9%. Pregnant women in the GDM group had higher Hemoglobin A1c(HbA1c), C-peptide, and homeostasis model assessment-insulin resistance (HOMA-IR) in late gestational period. Neonates born to mothers in the GDM group were at a higher risk of being large for gestational age (OR: 3.665, 95%CI: 1.006-11.91) and were also more prone to neonatal hyperinsulinemia (OR: 3.652, 95%CI: 1.152-10.533).Early onset GDM diagnosed by OGTT at 18-20 gws is associated with maternal and neonatal metabolic disorders and adverse pregnancy outcomes. Further randomized control trials on the therapeutic efficacy for early onset GDM will confirm the significance of early screen for GDM.