Introduction: Gestational diabetes mellitus (GDM) is diabetes occurring in the second or third trimester of pregnancy that is not clearly overt diabetes prior to gestation. Some ethnic groups, including Africans, Latinos, Asians and Pacific Islanders have the highest risk of GDM. Aim: The aim of this study was to evaluate pregnancy complications in patients with GDM belonging to either high or low risk ethnic groups. Methods. N=415 patients with GDM were enrolled, n=152 of high-risk ethnicity (HR-GDM) and n=263 of low-risk ethnicity (LR-GDM). Exclusion criteria were: age <18 years, pre-gestational diabetes, multiple pregnancy, psychiatric diseases and drug or alcohol abuse. A complete medical history and obstetrical information were obtained. Clinical and biochemical parameters were collected until delivery, as well as fetal ultrasound parameters. Results: LR-GDM patients were significantly older than HR-GDM patients (35.7±5.0 vs 31.6±5.1 years, p<0.001) and had higher third trimester BMI (30.5±6.6 vs 28.9±5.7 kg/m2, p=0.035). Prevalence of family history of type 2 diabetes, previous GDM and multiparity was higher in HR-GDM group than in LR-GDM group. Fasting plasma glucose at oral glucose tolerance test (24-28 weeks) and glycosylated haemoglobin at third trimester of gestation were higher in HR-GDM than LR-GDM (91.1±11.1 vs 87.5±11.7 mg/dl, p=0.01 and 5.5±0.4 vs 5.3±0.5%, p=0.001, respectively). A greater proportion of HR-GDM patients required insulin therapy during gestation compared with LR-GDM patients (54.7% vs 35.7%, p<0.001). In multivariate analysis, HR-GDM was an independent predictor of insulin therapy (OR 2.19 [1.1-4.45] 95%CI, p=0.03). Gestational age at delivery was lower in HR-GDM group than in LR-GDM group (38.1±1.9 vs 38.6±1.7 weeks, p=0.026). Furthermore, HR-GDM group had higher prevalence of preterm delivery (28.1% vs 15.5%, p=0.03) and emergency Caesarean section compared with LR-GDM group (34% vs 19%, p=0.008). In multivariate analysis, HR-GDM was independently associated with preterm birth (OR 3.07 [1.50-6.30] 95%CI, p=0.002) and emergency Caesarean section (OR 2.41 [1.28-4.55] 95%CI, p=0.007). Conclusions: Differences in pregnancy outcomes have emerged in patients with GDM according to different ethnic groups, with high risk ethnicities showing a greater risk of pregnancy complications. Specific ethnic groups might benefit from more tailored prevention, intervention and follow-up strategies in management of GDM.