To establish whether fasting glucose levels in the first trimester (FGFT)of pregnancy ≥ 92 mg/dL (5.1 mmol/L) (FGFT) anticipate the occurrence of maternal-fetal complications of gestational diabetes mellitus. To assess whether FGFT can replace diagnosis of GDM using the classical two-step oral glucose tolerance test (OGTT). A retrospective study of 1425 pregnancies with FGFT and O'Sullivan Test (OST) and/or OGTT according to OST results in the second trimester. FGFT sensitivity and specificity were assessed as compared to the conventional diagnosis of GDM. The relationship between maternal-fetal complications and FGFT was assessed in the total group and after excluding mothers who received specific medical treatment for GDM. Sensitivity and specificity of FGFT levels ≥ 92mg/dL were 46.4% and 88.8% as compared to diagnosis of GDM using Carpenter and Coustan criteria. In the total group, a statistically significant relationship was found between FGFT levels ≥ 92 mg/dL and newborn weight (3228±86 versus 3123±31g; P<.05), as well as a higher rate of macrosomia (6.9% versus 3.5%; P<.05). This association persisted after excluding patients diagnosed with and treated for GDM (weight: 3235±98 versus 3128±31 g; P<.05; percentage of macrosomia: 7.2% versus 3.4%; P<.05). FGFT is not a good substitute for conventional diagnosis of GDM in the second trimester. Pregnant women with FGFT levels ≥ 92 mg/dL, even with no subsequent diagnosis of GDM, are a risk group for fetal macrosomia and could benefit from dietary measures and physical exercise. Copyright © 2018 SEEN y SED. Publicado por Elsevier España, S.L.U. All rights reserved.