To determine if impaired glucose tolerance (IGT) impacts on the outcome of singleton pregnancies in Chinese women with a high (>26 kg/m2) body mass index (BMI), a retrospective case-control study was performed on 128 women with IGT and 128 controls with normal oral glucose tolerance test results, who were matched for pre-pregnancy BMI (within 0.1 kg/m2) and delivered within the same 3 year period. The IGT group was older, with more multiparae, a higher incidence of previous gestational diabetes mellitus, higher booking haemoglobin and fasting glucose concentrations, but no difference in the pre-pregnancy weight, gestational weight gain, or weight or BMI at delivery. There was no difference in the obstetric complications, mode of delivery, or the gestational age or mean infant birthweight. However, the birthweight ratio (relative to mean birthweight for gestation), incidence of large-for-gestational-age (birthweight >90th percentile) and macrosomic (birthweight ≥4000 g) infants, and treatment for neonatal jaundice, were significantly higher in the IGT group. The results suggest that some of the complications attributed to gestational diabetes mellitus are probably related to maternal weight excess/obesity in the affected subjects, but IGT could still affect infant birthweight outcome despite diet treatment which has normalized gestational weight gain.