This retrospective cohort study aimed to determine prevalence of GDM diagnosed before 24 weeks of gestation (early-onset GDM) and evaluate associated risk factors and compare pregnancy outcomes between different GDM status. A total of 1200 pregnant women attending antenatal clinic before 24 weeks of gestation were included. GDM screening was offered during first visit and repeat during 24-28 weeks of gestation, using 50-g GCT and 100-g OGTT. GDM was diagnosed in 110 women (13.9%) and early-onset GDM was found in 57 women (9.2%), which accounted for 65.9% of all GDM. Early-onset GDM had significant lower gestational weight gain and higher rates of preeclampsia, LGA infants, and NICU admission. Independent associated factors for early-onset GDM were age 30 years (aOR 4.89, 95%CI: 2.08-11.50, < .001), and previous GDM (aOR 12.26, 95%CI: 3.86-38.93, < .001) while DM in family was the only independent factor for late-onset GDM (aOR 2.53, 95%CI: 1.42-4.51, = .002). IMPACT STATEMENT Reported prevalence of early-onset GDM varies between studies, depending on the screening strategy and criteria used. Despite treatment, early-onset GDM has been associated with increased adverse maternal and neonatal outcomes in many previous studies. The risks associated with early-onset GDM and the evidence for benefit of early treatment are still unclear. The results showed that early-onset GDM accounted for majority (65.9%) of all GDM. Despite treatment, early-onset GDM increased risk of preeclampsia, LGA infants, and NICU admission. Independent associated factors for early-onset GDM were age 30 years, and previous GDM while DM in family was the only independent factor for late-onset GDM. Early GDM screening and intensive management, especially in high-risk women, should be implemented to minimise the risks of adverse outcomes. Further studies are needed to determine appropriate criteria to define early-onset GDM and to identify women at higher risk in different population and settings with different screening strategies. Effective management and approaches for this subgroup of GDM should also be further investigated.