OBJECTIVE: The purpose of this study was to identify independent determinants of mild gestational hyperglycemia (MGH) and gestational diabetes mellitus (GDM) and to assess the correlation between fasting glucose and C-peptide levels among control, MGH, and GDM women. RESEARCH DESIGN AND METHODS: A total of 1,022 consecutive women were evaluated with a 1-h 50-g glucose challenge test (GCT) at between 16 and 33 weeks of gestation. Women with a capillary whole-blood glucose > or =7.8 mmol/l in the GCT underwent a 3-h 100-g oral glucose tolerance test (OGTT). On the basis of a positive GCT, the women with a positive OGTT were classified as GDM, whereas the women with a negative OGTT were classified as MGH. The following data were collected for all women: age, prepregnancy BMI, ethnicity, clinical and obstetric history, pregnancy outcome, and C-peptide level. RESULTS: A total of 813 women (79.6%) were normal, 138 (13.5%) had MGH, and 71 (6.9%) had GDM. There was a stepwise significant increase in mean fasting glucose (3.6 +/- 0.4, 3.9 +/- 0.4, and 4.7 +/- 0.7 mmol/l, respectively) and C-peptide level (0.60 [0.1-2.4], 0.86 [0.3-2.0], and 1.00 [0.5-1.6] nmol/l, respectively) among the three diagnostic groups. Maternal age, non-Caucasian ethnicity, and prepregnancy BMI were associated with GDM, whereas only maternal age and prepregnancy BMI were associated with MGH. A positive correlation between levels of fasting glucose and C-peptide was found in control women (r = 0.39 [95% CI 0.31-0.46]). A similar result was seen in MGH women (r = 0.38 [95% CI 0.23-0.52]), whereas the correlation between fasting glucose and C-peptide was nearly lost in GDM women (r = 0.14 [CI -0.09 to 0.36]). The fasting C-peptide-to-glucose ratio was reduced by 60% in GDM patients versus control subjects and MGH patients (0.41 +/- 0.25 vs. 0.70 +/- 0.20 and 0.73 +/- 0.23, P < 0.001). CONCLUSIONS: Of the well-known independent determinants of GDM, only maternal age and prepregnancy BMI were associated with MGH. It appears that additional factors promoting loss of beta-cell function distinguish MGH from GDM. One of these factors appears to be ethnicity.
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