The lack of a glucose peak during the oral glucose tolerance test in pregnancy: what does it portend for perinatal outcomes?

Background: An univocal definition for a lack of glucose peak during the oral glucose tolerance test in pregnancy (flat curve) has never been agreed upon. Thus, the aim of this study was to provide a definition for the flat 75 g oral glucose tolerance test (OGTT) and to assess its clinical significance. Methods: A retrospective cohort study, where 8.810 pregnant singleton women were evaluated at the time of a 75 g OGTT between 240 and 286 weeks for the universal screening of gestational diabetes (GDM). The 75 g OGTT was considered flat when the difference between peak and fasting glucose concentrations was ≤30 mg/dL. A total of 953 (10.8%) women were diagnosed as having GDM, while 7.857 (89.2%) had normal glucose tolerance (NGT); 2791 women with normal glucose tolerance (35.5%) had a FLAT curve and 5066 (64.5%) had a concentration difference > 30 mg/dL (NGT). In all groups, we evaluated maternal characteristics and perinatal outcome. Results: Women with a FLAT curve were younger, taller, thinner, and their pre-pregnancy body mass index was lower than the other groups (all p < 0.001). The rate of obesity was also lower (p < 0.01). The vaginal delivery rate was higher than in NGT (80.4% vs. 77.8%; p < 0.01) and women with GDM (73.0%; p < 0.001) and that of primary cesarean lower than in NGT (11.9% vs. 14.8%; p < 0.001) and women with GDM (18.2%; p < 0.001). Between women with a FLAT and NGT OGTT curve, there was no significant difference for birthweight < 10th percentile (6.9% vs. 6.2%; p = 0.2), though the proportion of birthweight > 90th was lower (8% vs. 10%; p < 0.01). Conclusions: A 75 g flat OGTT as defined does not represent an abnormal maternal phenotype nor portend an adverse perinatal outcome.