Glucagon-like peptide 1 (GLP-1) levels may be reduced in type 2 diabetes but it has not been established whether a similar impairment exists in gestational diabetes mellitus (GDM). We studied this in a prospective cohort study of pregnant women (n=144) during oral glucose tolerance test (OGTT). GLP-1, glucose and insulin were sampled at 30-minute intervals during a 2-hour 75g OGTT and indices of insulin secretion and sensitivity calculated. In a nested case-control study, women with GDM (n=19) had 12% lower total GLP-1 secretion (area under the curve; AUC) compared to age, ethnicity and gestational-age matched controls (n=19), selected from within the lowest quartile of glucose120min values in our cohort. GDM had lower GLP-1 response in the first 30 minutes (19% lower GLP-130min and 17% lower AUC0to30min) after adjustment for possible confounders. Their glucose levels began to diverge at 30 minutes of the OGTT with increasing insulin levels, and by 120 minutes, their insulin levels were three times higher. In a secondary cohort of 57 women, which included 'high-normal' glucose120min values, low GLP-1 AUC0to30min was independently associated with lower indices of insulin secretion and sensitivity. In conclusion, we have observed that women with GDM have lower GLP-1 response at 30 minutes of an OGTT and hyperglycaemia at 120 minutes despite significant hyperinsulinaemia.