A 3-day diet containing at least 150 g carbohydrate per day has been used in many centers in preparation for the oral glucose tolerance test. The preparatory diet is thought to reduce false-positive diagnoses of gestational diabetes. The purpose of this study was to evaluate the necessity of a 3-day preparatory diet containing > or =150 g carbohydrate in otherwise healthy pregnant patients.Twenty healthy obstetric patients with abnormal results on a 1-hour glucose challenge test (> or =140 mg/dL) were enrolled in this prospective pilot study. Two oral glucose tolerance tests were administered. The first was with no dietary restrictions, and the second test was performed after a 3-day diet containing at least 150 g carbohydrate. Patients were given a 3-day dietary supplement, which contained 150 g carbohydrate per day. A food diary verified compliance with the diet and indicated other food intake.There was no difference in the number of oral glucose tolerance tests with abnormal results, with or without the diet (5 in each group). Additionally, no significant difference was found in the mean glucose values in the diet versus no-diet groups.A preparatory diet does not significantly alter the results of an oral glucose tolerance test administered to healthy pregnant women. The diet unnecessarily delays the diagnosis of gestational diabetes.