Previous studies indicate that fasting plasma glucose decreases during gestation, but the timing and extent are not consistent from study to study. We had an opportunity to examine this question in the normal pregnancy cohort of women studied in the Diabetes in Early Pregnancy Study. Subjects were monitored to identify pregnancy by human chorionic gonadotropin testing, enrolled within 21 days of conception, and screened to rule out gestational diabetes at the juncture of the second and third trimesters. All subjects were instructed to fast overnight for 10 to 12 hours. Three hundred sixty-one women were studied between 6 and 12 weeks of gestation. A median decrease in plasma glucose of 2 mg/dL was observed between weeks 6 and 10 (P=.007). In a smaller group of subjects evaluated through the third trimester, little further glucose reduction was observed. A reduction in glycosylated hemoglobin levels between 10 and 20 weeks (P=.002) followed the earlier reduction in first trimester glucose levels. Analysis by body mass index (BMI) showed a smaller first trimester reduction with increasing BMI, and none among severely obese women (BMI > 29.9 kg/m2). The decline in fasting plasma glucose in pregnancy begins early in the first trimester, well before fetal glucose requirements can contribute to the decline in the glucose level. Thereafter, plasma glucose levels decrease little. These results suggest that in the setting in which this study was performed (an overnight fast) maternal physiologic adjustments account for a reduction in plasma glucose early in the first trimester of pregnancy, and possibly even later in gestation as well.