Overt diabetes mellitus during pregnancy carries a significantly increased risk of adverse perinatal outcomes, but whether this is also true of less severe maternal hyperglycemia, or "gestational diabetes," remains uncertain. The Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study was planned to clarify the risk, if any, associated with less marked maternal glucose intolerance than is associated with overt diabetes. A standard oral glucose tolerance test using a 75-g dose of glucose was carried out at 24-32 weeks' gestation (target date: 28 weeks) in 25,505 pregnant women aged 18 and over at 15 centers in nine countries. Patient information remained blinded for 23,316 women whose fasting plasma glucose level did not exceed 105 mg/dL (5.8 mmol/L), and whose 2-hour plasma glucose was 200 mg/dL (11.1 mmol/L) or less.For women with blinded data, adjusted odds ratios (ORs) were calculated for adverse pregnancy outcomes associated with a 1-SD (standard deviation) increase in fasting plasma glucose (6.9 mg/dL or 0.4 mmol/L); a 1-SD increase in the 1-hour plasma glucose (30.9 mg/dL or 1.7 mmol/L); and a 1-SD increase in the 2-hour plasma glucose (23.5 mg/dL or 1.3 mmol/L). The respective ORs for birth weight above the 90 th percentile were 1.38 (95% confidence interval,

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