To examine the relationship between varying degrees of maternal hyperglycemia and pregnancy outcomes. This was a secondary analysis of a treatment trial for mild gestational diabetes including four cohorts: 1) 473 women with untreated mild gestational diabetes; 2) 256 women with a positive 50-g screen and one abnormal oral glucose tolerance test (OGTT) value; 3) 675 women with a positive screen and no abnormal OGTT values; and 4) 437 women with a normal 50-g screen. Groups were compared by test of trend for a composite perinatal outcome (neonatal hypoglycemia, hyperbilirubinemia, elevated cord C-peptide level, and perinatal trauma or death), frequency of large for gestational age neonates, shoulder dystocia, and pregnancy-related hypertension. Three-hour OGTT levels (fasting, 1-, 2-, and 3-hour) levels were divided into categories and analyzed for their relationship to perinatal and maternal outcomes. There were significant trends by glycemic status among the four cohorts for the composite and all other outcomes (P<.001). Analysis for trend according to OGTT categories showed an increasing relationship between fasting and all postload levels and the various outcomes (P<.05). Fasting glucose 90 mg/dL or greater and 1 hour 165 mg/dL or greater were associated with an increased risk for the composite outcome (odds ratios and 95% confidence intervals of 2.0 [1.03-4.15] and 1.46 [1.02-2.11] to 1.52 [1.08-2.15] for the fasting and 1 hour, respectively). A 1 hour glucose 150 mg/dL or greater was associated with an increased risk for large for gestational age (odds ratios, 1.8 [1.02-3.18] to 2.35 [1.35-4.14]); however, 2- and 3-hour glucose levels did not increase the risk for the composite or large for gestational age until well beyond current gestational diabetes diagnostic thresholds. A monotonic relationship exists between increasing maternal glycemia and perinatal morbidity. Current OGTT criteria require reevaluation in determining thresholds for the diagnosis and treatment of gestational diabetes. II. © 2011 Lippincott Williams & Wilkins, Inc.