There is an increasing incidence of type 2 diabetes mellitus (DM) among adolescents (especially females), and the serum glucose concentrations in pregnant women <25 years during a 3-h oral glucose tolerance test (3-h OGTT) seem to be lower than those of pregnant women >25 years. Among 115 Mexican pregnant adolescents (<18 years) we analyzed their serum glucose concentrations during: a) 1-h 50-g glucose challenge test (GCT) performed at 24-28 weeks of gestation (n = 103) or at 29-35 weeks of gestation (n = 12); b) A standard 3-h OGTT performed 3-5 days later. Eight adolescents had an abnormal GCT, three of whom also had an abnormal 3-h OGTT. Sixteen adolescents (13 with previously normal GCT) had an abnormal 3-h OGTT, 15 classified as GGI and one as gestational DM (GDM). Serum glucose concentrations in adolescents with GGI were higher than in adolescents with normal 3-h OGTT: a) at 60 and 120 min during the 3-h OGTT (p < 0.001); and b) when expressed as the area under the glucose curve (p < 0.001). Adolescents with GGI had serum glucose concentrations during the 3-h OGTT similar to adult, non-diabetic pregnant Mexican women. It is suggested that GGI in pregnant adolescents may represent an early sign of a future deterioration in glucose metabolism, leading to a higher risk for GDM in future pregnancies and/or type 2 DM in adulthood. Thus, the current criteria to diagnose GDM in adults may not completely apply to adolescents, especially in ethnic groups with high risk for glucose abnormalities and considering the frequency of multiparous adolescents, especially in developing countries.