Aim: The authors aimed to determine whether mild maternal glucose intolerance detected by abnormal screening by one-hour 50-gram glucose challenge test (GCT) and normal 100-gram oral glucose tolerance test (OGTT), which can be called as borderline hyperglycemia, is associated with increased risk of maternal and fetal adverse outcomes or not compared to normal and gestational diabetes mellitus (GDM) patients. Materials and Materials: Pregnant women with normal 50-gram GCT (198 cases), abnormal 50-gram GCT and normal 100-gram OGTT (160 cases), and impaired glucose tolerance (IGT) or GDM diagnosed with 100-gram OGTT (212 cases) were included. Data was collected from hospital automation system and clinical records. The authors compared demographic, obstetric, and neonatal outcomes among these three groups. Results: Mean maternal age (31.5 ± 5.2 years), history of GDM (4.2%), and the rate of cesarean section delivery in previous pregnancy (41,5%) were statistically higher in group 3 (IGT+GDM group) compared to both group 1 (normal 50-gram GCT) and group 2 (borderline hyperglycemia), respectively (p < 0.01, p < 0.01, and p < 0.01). The duration of maternal hospitalization was longer (2.40 ± 1.28 and 2.39 ± 1.25 vs. 1.79 ± 1.15 day, p = 0.001, p = 0.001, respectively) and postoperative hemoglobin values were lower (10.71 ± 1.44 and 10.69 ± 10.90 vs. 11.22 ± 1.43, p = 0.015, p = 0.006, respectively) both in groups 2 and 3 when compared with group 1. However preeclampsia was statistically more commonly developed in group 3 than in groups 1 and 2 (16% vs. 6.1% and 11.3%; p < 0.05). Neonatal hypoglycemia was more common both in groups 2 and 3 compared to group 1 (11.8%, 4.6% vs. 0%; p < 0.001, p = 0.045, respectively) and first minute apgar scores were higher in group 1 than in groups 2 and 3 (7.97 ± 0.55 vs. 7.65 ± 1.19 and 7.60 ± 1.17; p = 0.003, p = 0.001, respectively). Duration of hospitalization period for neonates was longer in groups 2 and 3 than in group 1 (2.22 ± 1.28 and 2.42 ± 1.48 vs. 1.82 ± 1.17 day; p = 0.006, p = 0.001). Conclusion: Borderline hyperglycemia can cause maternal, perinatal, and neonatal adverse outcomes. Both obstetricians and neonatologists must keep in mind the unfavorable pregnancy outcomes of borderline hyperglycemia cases and careful follow up is needed even if it is not accepted as GDM and IGT.