BackgroundTo evaluate the association between fasting glucose levels in women throughout pregnancy and the occurrence of gestational diabetes mellitus (GDM) and other pregnancy complications, macrosomia, and cesarean delivery.MethodsAn analytical cross-sectional study with 829 healthy pregnant women receiving health care at a public maternity unit in Rio de Janeiro between 1999 and 2008. The dependent variables assessed in the study were: GDM (was confirmed when two or more values were above the glucose curve using 100 g glucose), complications, mode of delivery and birth weight. Macrosomia was defined as a birth weight of >4000 g. The independent variables assessed were: maternal fasting glucose per trimester as a continuous variable, divided into three categories, socio-demographic data on the mothers. The level of statistical significance was set at 5%.ResultsThe mean fasting glucose levels of the women who had GDM were higher in the second trimester than for those who had no pregnancy complications (90.5 mg/dL vs. 78.5 mg/dL, p = 0.000). Higher mean fasting glucose levels were also found in the third trimester for women who developed GDM than for those with no pregnancy complications (90 mg/dL vs. 77.8 mg/dL, p = 0.016). Women who had a cesarean delivery had higher fasting glucose levels in the second (80.4 mg/dL vs. 78 mg/dL, post hoc = 0.034) and third (80.4 mg/dL and 77.1 mg/dL; post hoc = 0.005) trimesters than women who had a normal delivery. Also, higher fasting glucose levels were found in the second semester for women whose infants had macrosomia than for women whose newborns were normal weight (86.2 mg/dL and 78.8 mg/dL; post hoc = 0.003). The chance of develop GDM was higher for the women with glucose levels in the 90–94 mg/dL range in the second trimester (OR = 7.2; 95% CI = 2.33-22.24) than for the women whose glucose levels were in the <80 mg/dL and 80-90 mg/dL ranges.ConclusionSecond and third trimester fasting glucose levels below the cut-off values for the diagnosis of GDM are associated with an increased risk of pregnancy complications. The dependent variables assessed in the study were: GDM (present/absent), complications, mode of delivery (normal, forceps, cesarean), and birth weight.