ObjectiveTo evaluate fasting plasma glucose (FPG) as a screening test for gestational diabetes mellitus (GDM) among Mexican adolescents using International Association of Diabetes and Pregnancy Study Groups criteria.DesignRetrospective cohort study.SettingLevel-three medical institution in Mexico City.ParticipantsThe study population comprised 1061 adolescent women aged 12–19 years with singleton pregnancies, who underwent a 75 g oral glucose tolerance test (OGTT) between 11 and 35 weeks of gestation.Primary and secondary outcome measuresThe sensitivity (Sn), specificity (Sp), positive and negative predictive values (PPV and NPV, respectively), and positive and negative likelihood ratios LR (+) and LR (−), respectively) with 95% CIs for selected FPG cut-off values were compared. Secondary measures were perinatal outcomes in women with and without GDM.ResultsGDM was present in 71 women (6.7%, 95% CI 5.3% to 8.4%). The performances of FPG at thresholds of ≥80 (4.5 mmol/L), 85 (4.7 mmol/L) and 90 mg/dL (5.0 mmol/L) were as follow (95% CI): Sn: 97% (89% to 99%), 94% (86% to 97%) and 91% (82% to 95%); Sp: 50% (47% to 53%), 79% (76% to 81%) and 97% (95% to 97%); PPV: 12% (9% to 15%), 23% (18% to 28%) and 64% (54% to 73%); NPV: 99% (98.5% to 99.9%) for all three cut-offs; LR (+): 1.9 (1.8 to 2.1), 4.3 (3.8 to 5.0) and 26.7 (18.8 to 37.1) and LR (−): 0.06 (0.02 to 0.23), 0.07 (0.03 to 0.19) and 0.09 (0.04 to 0.19), respectively. No significant differences in perinatal outcomes were found between adolescents with and without GDM.ConclusionsAn FPG cut-off of ≥90 mg/dL (5.0 mmol/L) is ideal for GDM screening in Mexican adolescent women. An FPG threshold of 90 mg/dL would miss 6 (8.5%) women with GDM, pick up 34 (3.4%) women without GDM and avoid 962 (90.7%) OGTTs.